"Yeah, like I'm just gonna believe you have a twin sister that none of us have ever seen before."
“Pinkie! Wake up!”
The harsh demand was followed almost immediately by the sheets being ripped from Pinkie's bed, pulling her along with them. She hung in the air for a moment, spinning comically despite – or perhaps because of – her persistent slumber, and then landed on the floor on her back with a rather loud thud.
“H-huh?” she murmured, finally opening her eyes to see the bluish-gray form of her eldest sister glaring down at her. “Oh! Limey!” she said, instantly perking up and jumping to her feet. “You're here! I wasn't expecting you until tomorrow!”
“Yeah, well, Mom and Dad said they were 'worried for my health and the bonds of my soul',” Limestone ranted, making judicious use of air quotes, “and that I 'needn't spend every moment of my life at the quarry'.” She crossed her arms and grunted, blowing the sideswept portion of her gray undercut away from her face. She was already dressed in a white t-shirt, heavy denim overalls, and dangerous-looking boots. “It's like... running the quarry is my life. Aside from seeing my family now and then I don't need to spend time anywhere else. But no, they practically forced me to take an extra day off. I was already gonna take off tomorrow!” She raised her arms in frustration, too lost in herself to notice Pinkie watching her... or Pinkie getting dressed. “Heck, Maud isn't even here yet, and besides I still have loads of stuff to do at the quarry! But they just don't understand, it's all so...” She clenched her fists and teeth and let out an angry growl.
Then she turned around and saw Pinkie halfway into putting on her jeans. Pinkie stared back at her. “Right,” Limestone said, her frustration draining away into a sort of background gruffness. “Anyway. Marble made pancakes.”
Pinkie gasped in excitement, her eyes wide. “She did?! That's awesome, her pancakes are the best!” She quickly finished putting on her pants, then darted out her bedroom door.
Moments later she leaned back in and gave her eldest sister a serious look. “By the way? Mom and Dad kiiiinda have a point. You spend too much time in the quarry and you'll get sick cause of rock dust in your lungs,” she mimed choking on something, “just like great-Uncle Sedimentary Striation Pie!”
“Hey, I take precautions for that!” Limestone shot back, stalking angrily towards her unperturbed sister. “The quarry is way safer than it was back then!”
“Truuuuue,” Pinkie conceded with a cock of her head and a mischievous look, “but you don't take precautions against not having many friends! I mean, I'm pretty sure you're not even friends with the other people who work with you!”
“That's because they're my underlings,” Limestone said with a scowl, leaving the bedroom and walking down the hall with her skipping sister. “We're not supposed to be friends!” After making a few hand gestures that were no doubt meant to emphasize that fact, Limestone took one look at her smiling sister and shoved her hands into her pockets with a grumble.
“Sounds like a certain Grumpy Gus I know could use some more fri~ends!” Pinkie said in a singsong voice, leaning into her sister and grinning.
Limestone snorted as they rounded a corner and started heading down the stairs. “Whatever. If I ever actually get lonely maybe I'll actually consider it.”
Pinkie giggled. “I know, I know. I'm just teasing you.”
Limestone cracked a small smile. “Obviously. So,” she asked as they reached the bottom of the stairs and turned towards the kitchen, the smell of pancakes and syrup wafting through the hall and catching Pinkie's attention. “You gonna come out to the quarry again anytime soon? I got a few jobs for you and those, uh, exploding sprinkles of yours.”
“Hrmm... I guess so...,” Pinkie said with a thoughtful frown. “But I don't want it to turn into like a regular thing. I love the quarry and all, but I don't wanna work there.”
Limestone huffed. “Yeah, yeah, I got it. You all have your own dreams, I can run the family business by myself.”
Pinkie giggled again, and skipped ahead into the kitchen. She took another whiff of the air, then zipped over to the table just as her only-a-few-minutes-younger sister set down a platter of pancakes. “Goooooood morning, Marble!” she greeted, hugging her thoroughly gray-with-a-hint-of-green sister from behind.
Marble squeaked in shock, bolting upright and dropping the spatula she'd been holding. Pinkie quickly leaned around her and caught it, and grinned at her twin. “...morning...,” Marble said in a meek voice barely above a whisper, pushing away the bangs that covered her right eye. They fell back into place the moment she turned her head.
“Are you totally excited cause all four Pie sisters are gonna be reunited for the first time in like forever when Maud comes back home from her geology trip tomorrow? Cause I'm totally excited!” Pinkie asked excitedly as she prepared a stack of several pancakes, syrups, whipped cream, and fruit. It only took her a few seconds, and she zipped over to the table and took her seat the moment it was done.
“...mm-hmm,” Marble answered, fixing a much smaller plate of pancakes – each one filled with darker swirls much like their creator's namesake – for herself.
“What are we even gonna be doing together?” Limestone asked as she sat down, pouring herself a glass of limeade.
“Oh, I dunno,” Pinkie replied between bites, her mouth already rimmed with whipped cream. “Shopping, hanging out, the usual. Maybe see if any of my friends are around.”
“Oh yeah, didn't you say they all have magic powers too?” Limestone asked, looking at her little sister with keen interest. “Maybe they could help out at the quarry too." Her look became stern, and she added, "As long as they do what they're told."
“Nah, I'm pretty sure they all have jobs already,” Pinkie said before stuffing her mouth with pancakes again. She put a finger to her chin and thought as she chewed, then with her mouth full and her words unintelligible added something to the effect of, "But since they haven't met you, maybe they just don't know how fun quarries are?"
Limestone and Marble understood her, at least. “Exactly,” Limestone said with an oddly dangerous smirk. “You bring them, I'll bring the hard hats.”
“Sounds fun!” Pinkie more-or-less said before swallowing yet another mouthful of breakfast. “Plus, they'll finally get to meet the other two Pie sisters!” she added, looking excitedly at the two girls sharing the table with her.
Limestone looked up at her, a single brow raised. She looked from Pinkie to Marble, who was quietly drawing a small happy face in syrup on a pancake. “Wait, what?” she asked Pinkie. “Hold on, how the heck have they not met Marble yet? You're in the same year at the same school!”
“Hmm...,” Pinkie murmured, twirling a lock of hair in her finger, “good question. Marble, do you always sit in the back of the classroom?”
“...mm-hmm...,” Marble answered with a bashful look.
“And do you never raise your hand or talk to anyone unless they talk to you first?” Pinkie added.
“.....mm-hmm.....,” Marble squeaked, retreating behind her own bangs.
“Then that explains everything!” Pinkie brightly declared. “Obviously they've just never noticed you for some reason!” she said, grabbing her younger twin's shoulder. Marble brushed her bangs out of her face, smiling lightly. “Come to think of it, I probably should've introduced you to them a long time ago,” Pinkie added, rubbing her chin and narrowing her eyes. “That probably would've stopped Twilight from thinking you don't actually exist. Heck, she even asked me if you were Bigfoot, which is totally ridiculous cause your feet are like totally normal-sized!”
Marble put a hand up to her mouth and giggled softly. “...mm-hmm...”
Limestone snorted with contempt, then cracked her knuckles, leaned back, and smirked. “Well then, I guess we'll just have to introduce ourselves tomorrow when we're out with you and Maud. Make that friend of yours learn real good how real we really are.”
“Mm!” Marble smiled and nodded. Then she paused a moment, froze up, and let out a single short squeak.
Pinkie and Limestone's faces fell. “Hey, don't worry!” Pinkie told her twin, giving the frozen Marble a gentle rub on the back. “The girls are all super nice, totally not scary like at all.”
“Plus, who knows if we'll even run into them anyway?” Limestone added. “You know where they're gonna be?” she asked Pinkie.
“Hmm... Well, let's see, tomorrow's Saturday, so...” Pinkie thought aloud. “Though I guess since it's summer vacation that doesn't really matter. Still... Fluttershy's gonna be at the animal shelter again I think. Sunset and Twilight said they had some kinda research project, so they're probably gonna be working on that... Rainbow Dash has been like totally obsessed all week with finding out that new superhero's secrets, so I can't imagine she'll do anything else... So, I guess that leaves AJ and Rarity? They never said they had plans but I don't know if they're just gonna stay home either.” She finished with a helpless shrug, and Marble whimpered again and looked down at her plate.
“Hey,” Limestone said to Marble, putting a hand on her shoulder and catching her eye. “You don't have to come with us tomorrow if you don't want. We don't wanna make you uncomfortable, and you don't have to prove anything to anyone.”
Marble just kept looking down, her sisters watching her expectantly. After several seconds of silence, though...Marble clenched her hands and nodded. “...no...,” she said, surprising her sisters. “...I'll come...”
Pinkie and Limestone shared a stunned look, the former's of which rapidly turned into a grin. “Eeeeee!” she squealed, drawing both sisters into a hug. “Tomorrow's gonna be so much fun!”
Author's Note
Honestly, Limestone and Marble need more love. I really enjoyed writing them and their dynamic with Pinkie and each other, it's just so...fun and positive 
And the fact that this is the Equestria Girls universe makes it even better!
This storyline is continued in Synchrony.
