The patter of porcelain echoed across the hollow hallways of Canterlot castle from deep within. Besides the guard ponies battling to keep their eyes from drooping to low, the only pony who would grace these empty labyrinthine passages at this ungodly hour would be the princess of said hours herself.

Luna, in the dead of night when the moon was content with loitering among the stars and most ponies were so deep in their sleep they had no need for Luna’s dream-scape escapades, took a little time to reward herself with a particular treat.

What made this particular sweet so much more so, however, rest in the fact that they were not her to indulge in at all. Rather, they were her very own sister’s. Sealed away in a special calico cat cookie jar, with a strongly worded note that these were Celestia’s cookies, bold and underlined.

That was the only line of defense guarding them. No locks, no magic sigils or runes to keep would be pilferer’s at bay. Quite foolish, Luna smirked. Perhaps she’d just never noticed her partaking in some of the chocolate chip treasures stored away. I mean, with her only taking one at a time in spaced out intervals there was probably no chance for her to suspect any subterfuge. It’s not like any of the guards would have the gall to dare stand in the way of the Princess of the Sun and her sweets.

So Luna, perfectly confident and content in her thievery, made short work of unscrewing the head of the cat grinning back at her and eyeing the trove of treats piled high before her.

Inside the jar a fresh batch of cookies greeted her with that ever so intoxicating aroma of sugary goodness. Seemed like these were just put in here today, each one of them dotted with heaping chunks of chocolate and mounds of nuts.

Luna unceremoniously levitated the first one she spied and let it break away into her maw. As soon as the crumbs flaked to and fro between her teeth, her eyes shot open as if the flavor slapped her flank.

Celestia must have either found a new baker, nothing she’s stocked this jar with has ever tasted quite this delightful…

Before her mind could recover from the sudden assault on her senses, the cookie floating above her had seemingly disappeared.

She levitated what was probably the sixth or seventh cookie to her readily waiting mouth, but as it flew from the jar, her horn fizzled out and the cookie shattered into several crumby shards.

Rather than dwell on what would in most circumstances would be a troubling occurrence, she began licking every last bit of the fractured baked good off the floor. Come to think of it, how did she get on the floor in the first place? Her hooves felt a bit funny, like they were much heavier and fuzzy all over. They even looked slightly thicker, trading their slim contours for a bit of a rotund shape.

But that all had nothing to do with the cookies. Luna looked back up, her tongue still lapping up the cold floor despite there not being a spec left, only to see the cookie jar now tipped over onto the floor with its contents spilled over slightly

She wiggled herself forward with all her might, stuffing her snout into the the opening and shoveling more cookies down her throat.

The urge to satisfy her sweet tooth outweighed every synapse in her brain, so much so that she completely failed to notice that for the past dozen cookies that the lights had flickered on and a tall white alicorn stared straight at her gluttonous gorging.

“My oh my, looks like a certain pony was caught with her hand, or should I say head, in the cookie jar.”

“Sister, I swear this is not what it appears to be.”

Luna’s voice bounced off the walls of the jar, sounding far more slurred than shocked.

“Well you appear to be licking my cookie jar clean of crumbs. Which is very funny, because I could have sworn today I just filled it up to the brim earlier today. I wonder what could have happened?”

She answered only with the continued munching of cookies. She made no attempts to stop herself, letting whatever loud moaning and guttural sounds of satisfaction out as she pleased.

“Perhaps with have a rat problem in the house?”

“Exactly what we were thinking.”

A fierce burp sputtered out of her still full mouth.

“That is precisely why we are investigating for clues in your cookie jar.”

“With your tongue. Right. Well, I think I know exactly how we can sniff out this little rat of our, sweet sister.”

“We’re all ears. We’ll do whatever we can to aid you in righting this wrong.”

With the interior of the jar now scrubbed spotless, Luna plopped her head out of the container and began sniffing the ground for any other stray morsels that might have eluded her. Her nose certainly had a hold of her, guiding her along the ground until she bumped into Celestia’s royal fuzzy slippers. She looked up, seeing the object of her desire floating just out of reach encased in a yellow glow.

With an audible snap, her jaw jutted upwards like a shark leaping for a stray seagull floating too close the the ocean’s surface, but the cookie flew ever so slightly out of reach.

“Would you look at that, Luna, I think I’ve found our little cookie thief. Though I don’t think they’re a rat. Looks to me more like little piggy.”

“How dare you sister, we are not at all pig like. Retract your claim at once.”

“Well if you say so. I wasn’t mad, I was actually going to give this little snack to you if you really begged like the pig you are, but I guess I’ll just have it instead.”

Luna, still chomping at the bit to get past Celestia’s cookie keep away, began to squeal away with her best swine impression. She even went as far as to roll around in an imaginary mud pit on top of her incessant pleading. And just like that she was rewarded as she finally caught her rightfully earned treat.

“See, that wasn’t so hard to tell the truth. You do what I say and all the cookies you could ever want are you’re to chow down on to that pudgy little tummy of yours content.”

“Oh, but it looks like some poor little princess is all out of cookies.”

‘It’s a good thing big sister Tia bought at least three more cookie jar’s worth.”

Luna scuttled along the floor as Celestia revealed a large basket of those irresistible baked goods corrupting her sister’s mind.

“To get your first cookie, all you need to do is answer some simple questions.”

“You’re not just a piggy princess, but you’re a…”

Celestia now levitated a bib with “Cookie Monster” stitched on it, as well as a matching set of booties and bonnet.

“A baby piggy princess!”

Her voice began to assume a more infantile inflection, babbling away as her big sister adorned her with her new attire.

“Excellent. Now why don’t you show me how baby piggies act? Prove to me just how much of a hungry little porker you are.”

Luna offered up an encore of her previous pig performance. She rolled around some more, squealing away and even sticking her ass in the air and shaking her tail as if it were a little swirly tail. She even used her hoof to lift her nose up to complete the look.

“You’re doing wonderful little Lulu. But something’s not quite right. It seems like your outfit is missing a little something. Can you tell me what foals always have to wear?”

“DIAPERS!”

The sun princess smirked. She hadn’t even pulled out the package of diapers she had been concealing. Luna’s now blank canvas of a brain grabbed a hold of her suggestions far better than she anticipated.

Her big sister made quick work of amending the lack of proper padding, swiftly unfurling the diaper and powdering Luna’s ready and willing flank. A few firm pats of her butt sent faint puffs of white clouds out. The now fully kitted out foal mimicked big sister Tia by drumming on her diaper enthusiastically as she cooed to herself.

With Celestia thoroughly amused at Luna’s cookie addiction induced embarrassment, she spared no time preceding into the next phase of her planned punishment.

“I think my little sissy is ready for the rest of your treats. What do you think, diaper butt?”

Luna, too far removed from anything but getting her next fix, simply nodded on cue like a perfectly trained puppy.

“I can’t let my baby sister get too plump, so make a little room in your belly for all your new treats. That diaper of you’re looks like it got plenty of room.”

Celestia magically tugged at the waistbands of her sister’s diaper, hiking them up nice and high over her bulbous belly so that they fit nice and snug on her thickened waist.

“And, thanks to my little enchantment, you can go as much as you’d like so long as you keep eating.”

“So I think you know what you need to do, don’t you?”

Without a single moment of hesitation, Luna pushed with every fiber of her being. It’s a good thing Celestia ordered the guards patrolling this area of the castle to go on break for while, as anypony in the vicinity would have surely heard Luna grunting and cooing like the foal she had been reduced to.

Sure enough her efforts were rewarded promptly with a growing bulge at the seat of her padding. Much liker her mind, her body too fully succumbed to whatever spells bent Luna’s will to her sister’s whims. Her bowels had no trouble filling up her diaper far beyond a normal load. The solid mass of shit coiled away, not too soft but not too hard, just the perfect consistency for smushing on her tush.

Naturally, her bladder too joined the fray, letting the absorbent cores work a magic of their own as her padding swelled up as plump as the princess’s pudge it so comfortably swaddled. Luna pressed her hoof against the front. The fervent stream of hot piss pooled at the center of mass along the interior as the diaper took its sweet time dealing with the persistent flood. The golden nectar pouring out softened up her shit even more as it awaited the firm press of her hooves.

Finished with her duties and pleased with the doody causing her padding to sag, she let her big butt hit the ground, further cementing the damage done to her diapered rump. She bounced around, proudly reveling in the aftermath.

“Poopies for cookies! Poopies for cookies! Poopies for cookies!”

Celestia cringed a tad, both by how the second hand embarrassment from Luna’s chanting and the sheer smell that wafted toward her as her sister’s shit settled.

“Yes, yes, poopies for cookies.”

She threw a cookie into the air and Luna hopped to snatch it up. Her diaper audibly squelched as her flank hit the ground.

“But we can’t have cookies without milkies, can we now?”

She laid down a large sippy cup filled to the brim with perfectly chilled milk. Naturally, the gluttonous princess chugged as much as the baby proof cup would allow, craning her head all the way back to let gravity do all the work.

“Eat up Lulu, you still have plenty room in your tummy for more. And plenty more in your diapey by the looks of it.”

Celestia levitated cookie after cookie for Luna to snatch up and gobble down, only pausing when the little padded princess decided to chug down more milk and refill her sister’s sippy when it was empty from a gallon jug she brought from the fridge.

She rocked on her plump rear, smushing around the fresh mess and continually adding to it as her body saw fit. The only thing on her mind was eating more cookies, and if further filling up her already soaking wet and extra poopy diaper guaranteed her more, she’d make sure every inch of its interior was completely ravaged.

Her belly immediately ballooned further out to make room for the creamy liquid washing down bite after sweet bite. Her body now the perfect pudgey proportion with that little bit of extra chub that a sloshy tummy provided on top of her already chunky appearance.

The sun princess smiled all the while. The sister she knew so well sat reduced to a drooling infant obsessed with treats. She didn’t hold back in playing the role of Luna’s big sister, patting her head and combing her mane as she ate, cooing and encouraging her.

“Good girl, such a good stinky little girl. Don’t stop now, make your big baby tummy all warm and happy.”

She preceded to massage her sisters’ bulging belly, helping the magical digestion of the cookies along as it traveled to it’s diaper destination.

“C’mon, make more poopies for big sissy, let it all out. Just like a good baby princess.”

Luna’s ravenous appetite began to show signs of stopping just as Celestia realized she was down to her last cookie. Burps and drooling sputtering wiggled out of her, trying to catch her breath.

“Just one more, you know you want it. Eat it up for Tia.”

She chewed it up slowly, having trouble attempting to swallow it. Celestia lent a hoof by taking the gallon of milk and draining it down her throat.

She strained herself to chug it down, but slowly it swirled into her mouth until Luna’s rhythmic suckling stopped.

With a burp loud enough to wake the entire castle Luna finally had her fill. Finally free from the grip of the cookie cravings, her head began to swirl.

“No more… Cookie. I full… Thank you Tia.”

And with those final parting words, she laid her head on the ground and drifted asleep.

Celestia couldn’t just let her baby sister sleep on the cold hard floor of the kitchen. Luckily she had a pillow and a blanket at the ready. She plopped the pièce de résistance to complete the scene, a pastel purple pacifier, straight into Luna’s readily accepting mouth. She trotted back a bit to admire the full extent of her handiwork.

The once regal and majestic Princess of the Night now lay on the floor of the royal kitchen, surrounded by traces of cookies. Drool with flecks of the treats served as evidence of her gluttonous tirade as it flowed out of her mouth and onto the ground. Her slim figure fit to grace any would be fashion magazine or fancy gala now a pudgy poster pony for a late night ad claiming the best way to shed that stubborn belly fat. And last, but certainly the first thing anypony would notice, her fat diaper drooping against the weight of it’s filthy content and in dire need of a change that would surely not come until morning.

And while she could simply sit her and admire her lovely little sister’s current state all night long, a few pictures were sure to last much longer and serve as wonderful blackmail material to have under her sleeve to boot. She procured a small Polaroid camera to solidify this memory.

Between each flash, Celestia did take time to ponder if she perhaps went to far. Luna was nothing but a drooling mess, now trained to poop her diapers on command if it’ll get her more of her beloved cookies.

Sure tomorrow she’d be back to normal after a long recovery sleep. To a certain extent anyways, assuming the coast is clear of any guards or maids until she does. The extra pounds were sure to shed themselves in a week or two or three. And perhaps Luna would have problems making it to the bathroom for some time. And there was always the chance that she could relapse at the sight of a cookie. Which might lead to a mess for the royal custodians to take care of and mandatory diapering for at least a month.

At first she’d try to brush it off as a weird dream, but sooner or later she’d realize what went on here tonight. Pranks and punishments like this only go unnoticed for so long. I mean, what fun would they be if she didn’t have to deal with the embarrassment of looking over these photos and having her memory all flood back to her at once.

But knowing her she’d be plotting a revenge scheme and boy it’ll be a doozy when the time comes. But all in all, at least the contents of her cookie jar would be safe for a long, long, while.

Author's Note

Still a bit of shorter fic. Still trying to get my groove back in general. I tried writing like three other fics before I hit a good stride with this one. Perhaps it's because I haven't done something with weight gain before so it felt a bit more refreshing or perhaps it's because it's a fic about the royal sisters that kept me going (love stories with them, surprised I haven't done more featuring them), but who knows.

I've kind of changed my writing process so I write more consistently and with less of a strain, but as a consequence a lot more ideas are left on the cutting room floor because I don't beat my head against a wall for a half assed perfectionism.

In the end though I think that might lead to a better quality of stuff so it might be better that way.

But all that aside I hope you enjoyed this one, let me know what you think, comments and feedback are always appreciated.