Limestone's Lesson

Marble Pie had no idea what to say, since her mind felt just as incapacitated as her bound hooves. She could hear Pinkie Pie groaning softly in the bathroom, most likely still clinging to the toilet after filling her diaper in both ways. Marble tried her hardest not to glance back in the bathroom and see how her sister was holding up; although, the stench alone was more than enough to ensure she couldn’t turn her head. In addition, Limestone had a tight grip with a foreleg around Marble’s back, just above that sagging diaper wrapped around her waist.

“... uuuhhhhh…” Marble Pie almost didn’t realize she made that noise herself, or that her muzzle was slightly agape. Limestone leaned in a little closer against her sister’s side, not showing any revulsion or hesitancy when it came to Marble’s soggy padding. The mare was still shuddering a little from the warm diaper nestled against her marehood, as well as how exposed she felt with the spreader bar wedged between her hooves. But despite how overwhelmed she may have felt -- both from embarrassment and worry for what she was about to do -- Marble Pie eventually breathed out and tried to ask feebly, “Are… A-Are you… going to go change Pinkie?”

“Yes,” stated Limestone with an affirmative nod. “After you make use of your diaper as well~”

That clarification caused Marble to groan silently and hang her head. Even though she should’ve known her sister would include such a stipulation, she really didn’t want Pinkie Pie to wallow in her own filth all afternoon. If anything, not going herself would just add a heavy twinge of guilt the longer Pinkie was forced to wear her used diaper for Limestone’s perverted amusement. Of course, Marble’s eyes widened when she felt a sudden, sharp contortion of her bowels that made her body lock up. “Mmphh!!”

“Ooh, I think I know what that noise means!” Limestone beamed excitedly down at her sister, who was trying her hardest to turn her head away in an attempt to conceal her deep blush. Marble Pie’s muzzle was firmly clenched, but her sister could still hear her faint whimpering as she stood weakly. Since she wasn’t able to move around her spreader bar, she was struggling not to bend her knees in any way. But since she wasn’t looking up at Limestone’s insistent grin, there was nothing to prepare Marble for when one of her hooves gripped the rear of her diaper tightly.

“MMMPHHHH!!”

“Yeah, there we go…” Limestone shivered with her bottom lip bitten enviously, and smirked when she felt her sister’s wet and warm padding. Her hoof nestled in between the mare’s spread legs, leaving Marble completely vulnerable to the sensation of her marehood being put under that piss-soaked stimulation once more. She wanted to lift one of her forelegs off the floor to bite down on her hoof; unfortunately, her spread-apart stance ensured that she couldn’t do such a thing without falling face-first to the floor. So instead, all that she could do was struggle to keep her muzzle shut while squirming against her sister’s grip. All the while, Limestone purred sensually into Marble’s ear to make her shudder. “Mmmmm… How does that feel, sis? You like how warm it is?~”

Marble Pie whimpered more pathetically with her head hung low, and her lips quivering from the strain of resistance. Her eyelids may have been shut, but she could feel how badly her eyes were rolling back from the immensely tantalizing sensations hitting her most sensitive regions. She wouldn’t dare admit that Limestone’s inappropriate groping was doing anything for her, especially after seeing what became of their sister after she couldn’t hold it on. However, Marble Pie’s fur was standing on end like a petrified housecat’s as she writhed on her hooves and struggled to stand rigidly.

“Hmmmmm…” Limestone could tell how badly her sister was trying to resist the titillating feelings coursing through her system. But at the same time, the mare knew from personal experience how badly Marble’s nerves were most likely tingling while being rubbed from behind. She leaned in to slip her hoof even deeper between Marble’s legs, where every hard press elicited audible squishes instead of the dry crinkles further back. Limestone could feel the additional warmth of the mare’s piss-soaked padding right up against her crotch; meanwhile, her sister’s relentless pressure was causing Marble’s knees to buckle under her own weight.

“Ooh, you really soaked this, didn’t ya?~” Limestone snickered from the withheld groans that were muffled behind Marble’s tightly-pursed lips. She didn’t relent on her tedious hoof-flicking, which was grinding hard against Marble’s nethers and leaving her completely at Limestone’s mercy. If she wasn’t so preoccupied with keeping her knees off the floor, she likely would’ve allowed herself to openly moan from the tantalizing rubs that were causing her hips to contort against her sister’s hoof. Heck, considering how badly her stomach was starting to churn from all those sugar-free gummies earlier, Marble wasn’t sure if it was better to deal with that or Limestone’s jeering voice. “Mmmmmm… Just imagine how much better you’ll feel when you use it the other way too~”

“NFFFFF!!!” Marble Pie lurched down to the floor and squirmed even harder, with her hind-legs remaining as straight as could be in pure desperation. The growing pressure in her bowels was becoming harder to ignore, despite how well Limestone’s hoof was grinding against her marehood and distracting her conflicting emotions. Marble wished she could find a way to pull away from her sister’s forced embrace, just so she could find a toilet that wasn’t tied around her waist. But alas, the most she could do was try to keep herself from humping the floor in her wet diaper like Pinkie had done earlier.

“C’moooooonnnn, Marble…” Limestone licked her lips with anticipation, and took advantage of her sister’s more vulnerable position when she dropped to the floor. Since Marble’s padded ass was still raised up high, she used her other hoof to rest atop her back and keep her chin against the ground. With her other hoof, Limestone really dug in to rub and titillate the swollen crotch of Marble Pie’s warm padding. Marble wasn’t sure if it was the additional pressure that set her off, or the lingering fears of going in her diaper in the middle of the kitchen floor. But regardless, the mare eventually opened her muzzle just as Limestone leaned down and whispered into her ear, “I know you wanna go~”

“GnnnaaaAAAHHH!!!~” Her lumbering voice came out much louder than anything Limestone would’ve expected. But despite the mare’s raised brows, her grin widened quite a bit from hearing that first genuinely lustful response. Her hoof continued to grind and caress her sister’s padding, squishing against her nethers and making her shudder with every shivering breath. Meanwhile, Marble Pie couldn’t stop writhing on the floor with her muzzle wide-open, and her hooves skidding against the cold linoleum beneath her. “Aaaaahhh!! AAAAHHHH!!! MmmmmMMMMmmnnnghhhh…”

“Mmmffffff~ You seem to be enjoying yourself quite a bit there…” Limestone was growing antsier herself, but she made sure to keep both of her hooves on Marble before her “punishment” was complete. Eventually, Pinkie Pie was able to regain just enough energy to overcome her lingering shame, and waddle to the bathroom doorway with a look of humiliation and disgust on her face; however, she froze at the spot with a heavy blush, and her muzzle fully agape when she saw her sister being pinned down and groped mercilessly. Limestone may have been able to smell Pinkie from several feet away, but she kept her focus on Marble to continue molesting her saggy diaper and make her moan. “You know… I’m starting to think you don’t want your diaper off~”

“AHHHH!!” Marble lurched her head up from the floor with a terrified gasp. “N-NO!! I… I didn’t--”

“No, you don’t want your diaper off?” Limestone knew clear as day that her sister meant the word “No” in the exact opposite way. But as her smirk grew more perverse by the second, she was more than happy to say with malicious glee, “Well, if you wanna stay in that diaper all day, I’m sure that can be arra--”

“NO!!! AaaaAAAHHH!!” Marble’s shrill moans carried just enough fear as they did pleasure, as she struggled to find her voice between the harsh grinds of Limestone’s hoof. She struggled to breathe out quickly enough to regain her composure the tiniest bit; but alas, her voice still came out weak and shaky when she tried to protest. “P-Please, I… I d-d-don’t wanna--Nnnnnfffff~”

“What was that?~” Limestone’s mocking tone matched her sadistic smile perfectly, especially as she grinded against her sister’s padded crotch hard enough to silence her words. “You don’t wanna what, Marble? You don’t wanna use the toilet like a big girl?~”

“NO!! I wa--”

“Well, it’s settled then! Heh heh~” Limestone upped the pace of her flickering hoof, which kept anything but more strained moaning to belt out of Marble’s mouth. Pinkie was barely able to stand on her own hooves, let alone breathe while wearing a full diaper; because of that, she felt absolutely hopeless to stop Limestone from continuing her rampant assault on their sister’s soggy padding. Marble Pie eventually closed her eyes with another harsh shudder, and groaned out feebly as her marehood winked hard against the inside of her garment.

“AAAAHHHHH!!!~ MmmmmMMMMMmmmm…”

“Thaaaaaat’s right,” growled Limestone with a lecherous gleam in her eyes. “C’mon, Marble. Say it~ Say you don’t wanna use a toilet anymore. Maybe if you do, I’ll let you go…”

“AAHHHH!!~” The last thing Marble Pie wanted to do was admit something so heinous, even if it meant that Limestone would lay off her diaper-squishing and give her a moment to breathe. Unfortunately, the constant pressure held against her nethers (not to mention the internal strain she could feel from her churning gut) was leaving her head thoroughly clouded in a heavy fog of impulse and desperation. Her heart was racing from the anxiety of saying such words out loud, even though she knew she didn’t have to really mean them. But at the same time, she could only assume that by this point, Limestone’s twisted mindset would find some way to use those words against her regardless of any future consequences. “Mmmnnnnghhhh… Aaahhh…”

“What was that, sis?~” Limestone gave an especially harsh press to her sister’s crotch, which caused Marble Pie’s head to reel back and let out a heavier moan. Pinkie Pie could only stand in silence, too stunned to speak out against Limestone’s cruel form of titillation. All the while, Marble couldn’t ignore her sister’s taunting voice as she kept going. “Is there something you wanna tell me, Marble? Maybe if you did, I can let go of you for a little bit…”

“NNNFFFFF!!!~” Marble Pie couldn’t take it anymore. She didn’t want to think if it was the desperation of holding her straining bowels, or just the dire need of reprieve from her sister’s tantalizing touch. But regardless, she eventually threw her head up to shout in a shrill, distraught-sounding voice: “I… I don… I don’t want to use a toilet anymore!!!~”

Pinkie Pie cringed hard from the bathroom doorway, and slumped her head away in pity. Marble stood wide-eyed for a brief moment, looking completely stunned by the words that came out of her mouth. Meanwhile, Limestone merely huffed with a cocky smirk, and instantly let go of her shell-shocked sister. After pulling Marble back up so she was standing upright (albeit while shaking like a fallen leaf), Limestone casually trotted back towards her front so she could see her gracious smile. “See? Was that so hard to admit?~”

If Marble wasn’t so shocked and upset with herself, she likely would’ve given a strong nod of her head at the very least. But instead, all she could do was stand frozen while Limestone leaned in, and rewarded her with a small peck on the cheek. She then rested a hoof atop Marble’s shoulder, and grinned devilishly as she leaned in to add, “And just think… As soon as you let yourself go completely, I’m sure you’ll be able to say that with honesty~”

Marble Pie could only breathe out timidly, unable to hide the look of unbridled fear that left her face a garish pale. She wasn’t sure if Limestone really wanted her to feel that way, or if that was just another layer of humiliation for this ongoing punishment. But with Pinkie Pie standing nearby with the aftermath still being worn against her rump, Marble could only assume that her sister was going to remain firm on her standing no matter what. Was she actually going to follow through with keeping Marble in diapers after this punishment finally ends? Neither she or Pinkie had any idea.

Unfortunately, Marble wasn’t able to dwell on such a hallowing hypothetical for too long.

“Nnnnnghhh!!!” As soon as Limestone stopped rubbing her padded crotch, and her marehood wasn’t put under constant stimulation within her wet diaper, that strong sensation in her stomach came right back into focus. Marble tried not to lurch down in pain, but the building pressure was growing too intense to ignore. Her eyes clenched as tightly shut as her gritted teeth, and she tried to focus everything she had into keeping her tail from lifting up out of instinct. Meanwhile, Limestone just stood before her with a smug, confident grin in wait.

“Hmph~ You know it’s gonna happen eventually…” Limestone didn’t seem to worry about either of her sisters trying to escape, especially when both of their hind-legs were spread wide apart from their padding and bondage. Because of that, the mare casually exited the kitchen to acquire something out of sight. “Just wait there, little babies. I have something that I know will help Marble relax…”

Pinkie and Marble shuddered uncomfortably, and winced from how maliciously cheerful their sister’s voice sounded. Marble eventually turned her head to stare back at Pinkie, who was blushing hard after witnessing her sister’s harsh groping session. Since Pinkie’s backside was still hidden behind the bathroom doorway, Marble wasn’t able to see how brown and saggy the mare’s backside had become after her strenuous messing. She could certainly smell it well enough to make her snout wrinkle uncomfortably; but alas, Marble felt bad that she was grateful not to see it again. Fortunately, it seemed that Pinkie was more concerned with her sister’s well-being than her own.

“Marble? Are… Are you okay?”

Marble wasn’t sure what would constitute as being “okay” in such a twisted scenario. But at the same time, the mare felt calm enough to close her eyes and give a silent nod.

“Nnghhh…” Pinkie hung her head in shame, literally wallowing in her own filth as she struggled not to tear up in front of her sister. “I’m so sorry, Marble… I’m so sorry for all of this!”

“I-It’s not your fault!” Marble was surprised by how quickly that insistent reply came out. However, she was grateful to have said it with such sincerity to catch her sister off-guard. “I… I didn’t try to stop you. A-And I… I’m just as guilty…”

Marble hated admitting such a thing, but she knew it wasn’t a fact she could deny to either of them. She knew that Pinkie’s milk-indulgence would likely backfire, especially with all the work they had to do that day. Not to mention, joining in with her sister was definitely a stupid choice all things considered, even if Limestone didn’t choose to resort to such extreme measures. But before she could try to say anything else, Pinkie shot her a stern glare and stated firmly, “Marble, do NOT blame yourself for this! Neither of us deserve this, and you know that!”

“I know, I know. I just…” Marble squirmed a little as she shrugged and hung her head. Even though Limestone wasn’t pressing her diaper up against her crotch anymore, there was still enough pressure and warmth against her marehood to keep the fur along her back raised. She breathed out with the faintest shudder, and closed her eyes to keep from thinking about the repercussions of such a response to her wet diaper. After a moment, all she could do was breathe out and mutter, “I… I don’t know…”

Pinkie Pie wasn’t sure how to take that statement, especially while her sister stood with such a conflicted look on her face. She was sure that Marble was just reeling in confusion and shame, especially after being rubbed so relentlessly through her padding. But at the same time, Pinkie couldn’t help but wonder if her indirect answer was hinting at anything else…

“Alright, sis!~” Limestone trotted back into the kitchen excitedly, and was holding something that really got her sisters worried again. A long metal chain was hanging around the back of her neck, dangling over her chest like a punk-themed lanyard. As soon as she stepped past the doorway, Limestone gave an effortless twirl of one of the chain’s ends to make it swing up towards the top of the frame. Pinkie and Marble were stunned by how easily she was able to latch the end of the chain to a large metal hook that was screwed to the top of the doorway. When she let go of the chain’s other end, it was dangling a few feet off the ground, just barely touching the top of her head.

She then turned back to Marble with a more mischievous grin, and pulled out a pair of metal hoofcuffs with some pink fuzzy material around them. “Don’t worry, these are softer than they look~”

“Okay, WAIT A MINUTE!!” Even in her well-used garment, Pinkie was quick to glare at Limestone and rightfully scold her. “Limestone, this is going WAY too far!”

“Hmph~” Limestone completely ignored her sister’s surly state as she trotted up towards the frozen Marble. “It’s kinda hard to take you seriously when you’re like that, Poopy-Butt~”

Pinkie groaned intensely through her gritted teeth, her face turning a beet-shade of red. Unfortunately, she couldn’t exactly lunge in and attack Limestone when her gait was widened around the girth of her saggy padding. If anything, Pinkie was lucky enough to keep a sturdy grip against the kitchen floor, and not slip onto the linoleum for another dirty humping session. The mare took a moment to shudder uncomfortably, before looking back at her messy diaper and weighing her options; in the end, she felt far too humiliated and upset to even think of risking her position by moving too much.

“That’s what I thought~” Limestone nodded cockily to her sister’s silence, and walked up to Marble before she could try to escape. Marble gasped when her sister latched one of fuzzy cuffs around her hoof, before lifting it up to gently guide her towards the chain. Her rear hooves could only slide against the kitchen floor, not providing any grip or tension to counteract Limestone’s coaxing pull. Marble whimpered through her clenched muzzle, and blushed heavier the closer she was dragged towards the hanging chain. Pinkie could barely move in her diaper, but she tried to carefully step forward inch-by-inch for her sister’s sake. But alas, Limestone was already grinning wide when she hoisted up Marble’s other foreleg. “Here we go!”

“EEP!!!” Marble Pie felt the other cuff latch around her hoof, binding both of her forelegs together while being lifted up by Limestone’s immense strength. The chain hanging from the doorway was clipped to the middle of Marble’s hoofcuffs, and was yanked up by Limestone like she was hoisting up a piece of farm equipment. Marble wasn’t able to stand under her own weight, both because of her spreader bar and the wet diaper swollen between her legs. But with the aid of Limestone’s tension on the chain, Marble soon found herself precariously hanging by her cuffs as they were raised high in the air, and latched to the hook above the doorway to keep her in place.

“Daaaaawwwww~” Limestone let go of the chain, and smiled with satisfaction when she saw how restrained her sister had become for this “punishment”. Not only was Marble Pie cuffed and hanging bipedally off the floor, but her hind-legs were spread wide apart to keep her from trying to clench. If it wasn’t her own sister hanging in such a vulnerable state, Limestone would’ve likely found the sight to be unbelievably hot. Of course, she was still blushing with a bitten lip as she walked past the doorway to admire the diapered mare from behind. “Nnffff~ Yeah, this’ll do wonderfully…”

Limestone took a second to glance back at Pinkie, and tried not to giggle with a scoffed smirk. The pink mare had to grip the side of the kitchen counter again, just so she couldn’t skid off her rear hooves and fall ass-first onto her mushy padding. Limestone thought to herself how easily it could’ve been to place butter or oil of some kind on the kitchen floor, just to see something like that happen to her sisters; however, she also knew that was way too dangerous to even joke about. She may have wanted them to endure her punishment, but she didn’t want to hurt them. That would’ve been wrong.

“Don’t worry, Pinkie. I’ll clean you up as soon as Marble finishes here~” Limestone turned away before Pinkie could even get the chance to say anything in response. The mare shuddered when she took a moment to admire Marble Pie hanging by her hooves off the floor, with that plush white padding pointed right at her. Limestone could see her sister’s tail flicking back and forth erratically, hitting the sides of the plastic with hard smacks. Limestone pondered her sister’s state for a moment, and eventually huffed with a contemplative grin. She went over to grab one of the keys she had hidden out of sight, and walked up towards Marble’s backside. “You know what? Since Pinkie got her pants off…”

Much to Marble and Pinkie’s shock, Limestone undid the padlock that was holding Marble Pie’s plastic pants over her diaper. The clear plastic was pulled down to her knees, leaving nothing to cover the sight of her bloated and yellowed diaper from view. “Besides,” added Limestone with a cheeky shrug, “it’s not like I’m too worried about you taking off that diaper right now~”

Marble Pie groaned and trembled in her bound state, and could feel the plastic pants wrapped around her spread knees. Since all four of her hooves were tightly restrained, and she was left completely immobile at her sister’s mercy, all she could do was hang her head down and struggle not to feel so embarrassed. The only saving grace she could think of was how she was indoors with only two other ponies present; but even then, Marble felt unbelievably exposed and humiliated in her wet diaper. Her heart couldn’t stop racing, and her face was as red as could be.

And worst of all, Marble could still feel that intense tingling from her marehood winking profusely against her soggy padding.

“Now then…” Limestone stood up on her hind-legs, and got right up behind her sister. She wrapped her forelegs around Marble’s torso, clinging to her chest while her crotch pressed up against the back of her diaper. She shuddered with a soft groan in elation, and bit her bottom lip while grinning excitedly. Meanwhile, Marble was trying not to get even hotter as she felt herself so helplessly trapped in her sister’s embrace. She tried to open her muzzle to protest, but all that could come out was a weak and shaky moan. The pressure building up in her bowels was becoming more painful, but she was trying her hardest not to mess herself while Limestone was right behind her. Unfortunately, as soon as she felt one of her sister’s hooves drift down towards her upset stomach, Marble had a bad feeling that she was wanting her to do exactly that.

“Mmmm… It feels like you have a lot built up in there~” Limestone gave a firm press of her hoof against Marble’s belly, causing the diapered mare to yelp out a heavier moan in response. Pinkie Pie couldn’t waddle over towards them in her current state, even as she tried to lean against the counter and skid a couple feet across the linoleum floor. All she could do was look away from the ongoing madness, but she could still hear her sister’s deep groans nonetheless. Marble Pie clenched her eyes shut and tried her hardest not to give in; unfortunately, that was much easier said than done while Limestone was continuously pushing her hoof into her constipated gut. “Yeeeaaaaahhhh… You really need to go, don’cha?~”

“Mmmnnnghhhh…” Marble Pie’s muzzle was trembling from how badly she needed to unload. Her tail was already lifting up quite a bit with every erratic wag. However, she was struggling to keep herself in control while counteracting her sister’s forceful pushing. Limestone used her other hoof to gently pet Marble’s chest, almost as if she was trying to relax and coax her at the same time. Marble couldn’t even clench her cheeks while her legs were spread apart, and she wasn’t sure how much longer she could handle all the building pressure inside of her.

“C’mon, there’s nothing wrong with answering a simple question~” Limestone gave her sister another gentle kiss to the nape of her neck, which was already becoming sweaty from how hard she was straining in her bondage. She gave a gentle rub to her sister’s belly before saying sweetly, “It’s alright, Marble. You can be honest with me… Do you need to go potty?~”

“Nnnnghhh!” Marble Pie lurched her head down as much and she could and breathed out hoarsely before giving a feeble nod. “... mmmm… mhmm.”

“That’s better~” Limestone decided to “reward” her sister with another hard prod of her hoof against her stomach, eliciting another deep moan that left Marble quivering in her chains. Her tail lifted up even more, with the tip still wagging about weakly. Marble was tensing up greatly in her chains and cuffs, but it could only do so much while standing in such an exposing state. And with Limestone keeping a tight grip around her, there was nothing for her to do but endure those intensifying bouts of pressure. “Now let me ask you this,” continued Limestone with a more devious grin while nestling up against her sister’s padded rump. “Mmmmm… Do you think you could reach the toilet in time if I let you go?~”

Marble Pie wanted to say “Yes” more than anything else. Heck, if she was uncuffed right that instant, she would’ve tried her absolute hardest to skid towards the bathroom and get her diaper off. However, the mare also knew that her sister’s “offer” was nothing more than a trap. Not only was the bathroom door locked once more, but she was sure that the toilet seat was still latched down with those blasted locks.

Because of that, Marble Pie merely groaned in defeat before giving a soft shake of her head. “Nnnffff… Nnn-Nnn…”

“Heh~” Limestone gave another couple harsh prods to her sister’s belly to keep her squirming. “That’s right. You can’t reach the toilet in time. All you can do is just take a deeeeeep breath, and use the potty you’re wearing right now. You already used it once, so what’s the big deal?~”

There were many “big deals” that both she and Pinkie could list off the top of their heads. Nevertheless, Marble Pie was too afflicted in shame and disgust to really think of anything but the building strain on her bowels. She was twitching hard in her bondage, and her raised tail was becoming more rigid with every weakening flick. She gritted her teeth tightly, but a shrill groan could still be heard to leave Limestone grinning from ear to ear. Marble hoped that her own strenuous noises could cover up the lewd whispers her sister was making into her ear; but regardless of her resistance, she was totally helpless when it came to Limestone’s coddling.

“Theeerrrrrrrre we go…” One of Limestone’s hooves pulled away from her chest, and she leaned back just enough to place it against the back of Marble’s diaper. The padded mare moaned out from the sensual press against her garment, as well as her sister’s other hoof rubbing her stomach in wait. Her breathing came out faint and rapid, and beads of sweat could be seen trickling down her blushing face. Limestone didn’t let up on the pressure she was delivering to Marble, especially when she could tell how hard she was struggling not to shit herself right then and there. “Nnnfffffff… C’mon, sis~ Show me you don’t wanna use that big, scary toilet anymore…”

Pinkie Pie was frozen while leaning against the counter, gawking at Limestone from behind like she had turned legitimately insane. Of course, it seemed highly likely that was the case with how eagerly the mare was trying to get Marble Pie to unload in her diaper. Her hoof was pressing into her stomach repeatedly, like she was trying to pump her insides and get her bowels working overtime. All the while, Marble’s shuddering moans intensified the more that building pressure caused her hooves to curl up in her cuffs. “AaaaaAAAAAHHHH!!!~”

“C’mon, Marble,” purred Limestone sensually. “Say what you wanna do~”

“I… I wa…” Marble Pie was too overwhelmed in desperation and embarrassment to realize what she was saying out-loud. Even though she technically needed to go, she ended up moaning out, “I wan… Iwannausemydiaper!!~”

“Oh, what was that?~” Limestone’s taunting tone matched her devilish grin perfectly. “You wanna do what in your diaper, sis?”

“I… Ineedtogo!!” Marble writhed and thrashed even harder in her bondage, but it was nothing Limestone couldn’t handle herself. While her hooves continued to rub her stomach and padding enticingly, Limestone waited with an antsy grin when her sister’s voice became more strained. Marble could only let out a harsh-sounding moan before she threw her head back, and her knees instinctually bent inward when her tail flagged up high. “AAAAHHH!!! I need to… I need to gooooOOOOOHHHHH!!!~”

Marble’s muzzle tried to close shut, but all she could do was grit her teeth before groaning with that first push. “NNNNNGHHHH!!!”

Marble Pie didn’t plan to push that first hot, steamy load directly into her diaper. If anything, it seemed that her body was working on its own accord to coincide with Limestone’s dirty scheme. Pinkie Pie couldn’t even look, and reeled away from the incoming mudslide that caused Marble’s padding to bloat out almost instantly. Since Limestone’s hoof was pressed right up against the formerly clean padding, the sheer mass of Marble’s load caused the crinkly plastic to push out around her hoof. Limestone groaned and shivered in absolute delight, relishing in the intense warmth and mushiness that she could feel while Marble filled her diaper uncontrollably.

“Aaaaahhhhhh!!!~” Marble Pie was trembling in her restraints, and felt utterly hopeless as she continued to push with all her might. More and more of her filth was filling up the diaper’s backside, which caused the bottom to hang even heavier between her legs as they did before. The formerly white plastic beneath her tail quickly morphed into a deep, garish shade of brown to match Pinkie’s rump a few feet away. Even though the relief of all that internal pressure dissipating was enough to make her fur stand on end and her blush to deepen, it was accompanied with an unrelenting sense of shame and humiliation as she felt herself shitting in her sister’s embrace. “NnnnnNNNNnnnghhhh!!!”

“Mmmphhh!! Oh, that’s a GOOD girl~” Limestone looked absolutely elated with the end result, and made sure to rub the back of Marble’s saggy padding as it grew heavier and browner by the second. Much like with Pinkie’s unloading, it took well over a minute or two before Marble could let out a soft, quivering sigh of relief. Limestone eventually pulled herself off her sister’s back, and stood with a triumphant grin at the aftermath of Marble’s first messing. Both the mare and her full diaper could only hang lazily in the open doorway, much to Limestone’s approval.

“Oh, man,” she said with an excited shiver that made her beam with pride. “I can’t wait to tell Doctor Elder--”

“LIMESTONE!!!”

Due to all the insanity that had just occurred, none of the three mares seemed to overhear the front door opening up. Limestone turned around to see their parents -- Igneous Rock and Cloudy Quartz -- were standing outside the kitchen with their eyes and muzzles wide open in shock. Quartz’s travel bags fell to the floor, while Igneous’ left eye twitched in stunned silence.

Marble Pie froze up in her precarious bondage, wearing nothing but a mortified expression and her fully-used diaper. Pinkie Pie tried her hardest to slip her head away from their line of sight, not wanting them to see her in the same messy padding that her sister was wearing. Both of them were utterly terrified about their parents’ presence, while Limestone just stood without a shred of remorse or worry on her face. In fact, the mare just stood casually beside her bound sister, and smiled at her parents with a light shrug.

“Oh, hey Mom! Hey Dad! Don’t worry, I was just teaching these two babies what happens when they skip out on their chores. Whoo! You wouldn’t believe how silly they looked napping earlier~”

Igneous and Quartz just gawked at their daughter without a word escaping either of their open muzzles. After nearly a full minute of uncomfortably awkward silence, with neither of Limestone’s sisters speaking up as they wallowed in their filth and shame, the mare’s smile slowly dropped when she finally realized something was up.

“... uhhhh… did… did I do something wrong?”