It was one in the morning, and the sounds of a key scraping against a door's lock pierced the silence of vice-principal Luna's apartment living room. Eventually, the key was able to find it's mark, and the front door's handle started to turn. The incoherent mumbles and giggles of two women could be heard from the other side, increasing in volume as the door was finally pushed open.

"T-there we g-guh.. go" Vice Principal Luna burped as she took her first wobbly step into her apartment, supporting her older sister over her left shoulder.

"H-heh, you win the r-race home, Luna," Celestia giggled nonsensically.

"Sister, you are just so d-drunk right now." Luna murmured, giggling along with her sister as the two stumbled inside.

"Nooo, you is the drunkest p-person in this bar Lulu~" Celestia slurred, head wobbling side-to-side as she gave her sister a silly smirk that was probably meant to look sly. But as she was doing so, her face instead contorted into a scrunch. "H-hey Lulu?" She asked, sounding puzzled.

"Yea, sis- Erup! -ter?" Luna inquired.

"I gotta poooo" Celestia groaned, her left hand coming down to lazily pat her abdomen as she squirmed uncomfortably. "And it's a biiiggg one, heh."

"Ung, me t-too~" Luna groaned in reply, feeling a similar tightness that had been collecting in her lower intestine. "The bathroom's this way, c-come on."

Finally flicking on the light switch after three separate attempts, the two drunks stumbled together off into the hall from the living room, leaving the front door wide open in the process. The bathroom door lay open at the end of the hall, lights left on from before they left. Luna paused for a moment as she heard a quiet hissing sound. Looking down, she watched as dark wet stains were cascaded down the inner parts of Celestia's tight denim jeans, the light from the bathroom glistening off the rushing liquid as it gushed from her sister's crotch.

"H-hey, Celly! You're p-peein yourself -hic-." Luna pointed out dumbly.

"Mmm. It's s-so nice and warm~" Celestia hummed, eyes closed in bliss as her free hand came down to press against her piss-soaked crotch. It was still flowing out of her, and she could feel the force of her stream on her palm through the fabric.

"No, no, no. You're supposed to do it on the toilet, you -hic- dumb-ey ass" Luna scowled, attempting to roll her eyes though only half succeeding in the effort.

The two eventually reached the bathroom, Celestia having left a dark trail in the carpet as her bladder finally finished emptying itself into her pants. Luna carried her sister over to the toilet and sat her down on the edge of the bathtub. The principal quickly lost balance, falling back into the tub, though thankfully without hurting herself.

"I didn't know you bought a tub for me, Lulu. You're the bestest sister ever..." Celestia slurred, slumping back into the tub a bit more as Luna squinted at the toilet, trying to figure out the swirling blob that she was staring at.

*BRRAAAPPP*

A loud fart suddenly trumpeted from Celestia's rear, the sound leading the school principal to giggle like a toddler.

"My tummy's talkin~" She blurbed, a silly smile spread across her face, as she turned to the side and released yet another burst of noxious gas.

"C-come on Celestia, you gotta pull your pants down to use the t-toilet." Luna stammered, blinking rapidly as she was awoken by the stench of her sister's expulsions.

"Oh, yea..." Celestia sighed, reaching down and slowly and hooking her thumbs through the belt loops of her jeans. She'd had a belt on when they left earlier, though it seemed to be gone now. She pushed her pants down until they were bunched up around her ankles. Her legs felt sticky, smelling strongly of urine, and her once white silky panties were stained yellow. Feeling relaxed as she felt the cool bathroom air tickle her legs, Celestia sighed and laid her head back against the side of the tub.

*Splorptpr!*

Luna heard a strange sound and turned to watched as the seat of her sister's panties suddenly jutted outwards, deep dark brown stain pushing through the thin fabric as a rush of diarrhea flooded into them. Brown colored liquid dripped steadily from the center of a growing bulge, spilling out from the leg holes of her panties as well. Some rushed down towards the drain while the rest began to pool around her ass.

"Celly!" Luna whined. "You're supposed to poop in the toilet, dummy. You're p-pooping in your -hic- p-panties."

"Hmm, so nice and squishy, heh," Celestia hummed, cheeks reddening as she felt the building mound in her underwear meet her already heavily smeared ass cheeks.

"No, no, y-you're doin it all -hic- wrong! You're too d-drunk. Watch, -hic- I'll do it the right way~" Luna declared.

Popping up the lid of the toilet, Luna turned and, after yanking down her own jeans, lowered her ass down onto the cold porcelain seat.

"You're supposed to do it like this!" Luna grunted, face scrunching up as she began to push.

*Crackle, Pftttchk!*

Luna moaned in relief as she felt her bowels start to empty themselves, sighing as a series of firm slender logs slid out through her anus. At the same time, a warmth grew from her crotch as her bladder emptied itself as well. But after a few moments, Luna realized that something didn't feel quite right.

"Heh, you poopied panties too..." Celestia giggled drunkenly as she pointed at her sister, eyes fluttering closed a moment later as she then promptly passed out in the tub.

Staring down, Luna realized that she was still wearing her black lace panties, having actually only pulled down her pants.

"Oops~," Luna said, blushing as she stood up from the toilet in the middle of one of her logs.

A massive bulge sagged down behind her, it's mushy and warm contents resting against the back of her thighs. Not used to the new weight hanging from her ass, she suddenly lost balance and fell backward at an odd angle.

"Woah!" She gasped, thankfully missing the edge of the bathroom counter by mere inches as she landed butt-first onto the hard tile floor.

*SPLART!*

Soft creamy poop gushed from her panties upon impact, spreading all throughout the inside of them as her accident miraculously managed to completely break her fall. Moaning as she felt the warmth of her shit squishing into her crack and up through the gusset of her panties, Luna's eyes drifted shut as well.

The sounds of snoring filled the bathroom as the two fully grown women slept peacefully in their messy underwear. The following morning was sure to be a doozy...

Author's Note

This is what happens when two competitive sisters have a drinking contest after eating a full meal...