Author's Notes:

I did this chapter by request, kind of. (Someone wanted a CMC story for something a little different)

I hope the quality doesn't suffer from my lack of confidence. And don't worry, she's aged up! No incest either, although that does seem to be a thing.

Several years had passed since Rarity had found the crystal that had made her messy activities easier and more fun. Rarity was in her twenties, and still had her good looks.

She was in her backyard, wearing her light blue tank top and skintight purple shorts which were absolutely filthy, for she had already peed in them and soiled them, and was now playing around in the mud.

She had just let out a boisterous wet fart that she was sure had made her shorts and underwear even dirtier, when she suddenly heard a voice.

"Rarity?"

Rarity turned in the direction of the voice and blushed beet red, though no one could tell because of all the mud on her face.

Sweetie Belle stood there. She was as tall as Rarity - and just as beautiful - for she was now 18, a year younger than Rarity had been when she found the crystal. She was wearing a light blue T-shirt and skintight pink pants.

Rarity froze, accidentally farting loudly. How could she have forgotten her sister was coming over today? She could not believe she was seeing her in this state. Hopefully she didn't notice the smell.

"Sweetie Belle, this... this isn't what it looks like!" said Rarity. "I'm under the sirens' spell! I..." She paused as a hissing noise could be heard. Rarity was peeing, since she had cast a timed pee spell with the crystal, and she couldn't help but sigh in relief.

"So, you like to do that too?" said Sweetie Belle.

"What?"

"I think it's really fun to play in the mud, and I always pee myself before I start." She blushed a little.

"You do?" Rarity couldn't believe what she was hearing.

"And I also think it's fun to fart."

"So do I." She felt immense relief that Sweetie Belle didn't react with disgust or make fun of her. And that her bladder was about 3/4 empty, for she was still peeing. "It must run in the family."

"Can I play in the mud with you?"

"If you want. But... I may have soiled myself a bit... no, a lot, so this mud isn't just mud."

"That's all right, I don't mind. Too bad I don't have to pee."

"I can fix that. If you want."

"Really? How?"

"This magic crystal I found can make anyone have to use the bathroom in either way, or fart, as much as you want. It can also make any mud, dirt, or anything else like that disappear. The crystal also repels any form of dirt, so I never have to worry about it getting dirty."

Sweetie Belle's eyes were wide with excitement. "Do it right now!"

Rarity had never thought she would ever use the crystal on anyone other than herself, least of all her sister. But Sweetie Belle shared her interest and wanted this. So, Rarity held up the crystal and cast its spell on Sweetie Belle.

Immediately Sweetie Belle could feel a powerful pressure. Not in her bladder, but she enjoyed this. She let out a loud fart for about seven seconds.

"That wasn't too much, was it?" said Rarity.

"No, I love it!" said Sweetie Belle, farting again and slapping her butt playfully. "Except... why don't I have to pee?"

"It'll come a bit later." Rarity held up the crystal and farted again. "We should have a farting contest!"

"You're on!"

Rarity stood up, and both she and Sweetie Belle released a series of loud, wet farts. Each felt aroused by her own farts (but not her sister's). They farted so much at the time time that they couldn't tell whose farts were the loudest or longest, but they didn't care, it was so much fun!

PPPPRRRRRRT!

Sweetie Belle put a hand on her butt. The back of her skintight pants felt a little wet. She turned around to see a brown stain on her rear. She was a little surprised, but giggled, as she enjoyed it.

Then she could feel an urge to pee like nothing she had ever known. As far as she could remember she had never involuntarily wet herself, so there were levels of desperation she hadn't experienced.

"Wow, Rarity, this is... I haven't had to pee this badly since... well, ever!" She squealed as a bit of pee leaked out of her, forming a damp spot on her crotch.

Watching Sweetie Belle's motions made Rarity want more of it herself. She cast the spell on herself, and began peeing, washing the mud off her legs as the pee soaked her shorts and poured down on the mud below her.

The sight and sound of this was too much for Sweetie Belle, causing her to lose control. Or, maybe part of her voluntarily let go. She couldn't tell. She sighed in relief as she felt the warm wetness soak the crotch, legs, and butt of her skintight pants. In the middle of wetting herself she let out a loud fart, and both she and Rarity giggled, Rarity farting even louder.

After about three minutes, both Rarity and Sweetie Belle finished peeing themselves.

"I've never peed for so long," said Sweetie Belle, breathing heavily and satisfied beyond what she'd imagined. Her pants were absolutely drenched, and she could only imagine the state her light blue panties were in.

"Well, when you're ready, come in," said Rarity, and she sat down, farting as soon as her rear splashed into the mud, and she giggled.

After feeling how wet her pants were and taking in the moment, Sweetie Belle dove into the mud, splashing Rarity, who of course didn't mind.

She sat up. Her skintight pants were now brown and smelled bad, but she liked it that way.

The two girls played in the mud for about an hour, farting the entire time and occasionally peeing. They both took great pleasure in this, maybe more so in front of someone else, though none of their arousal came from the other doing it.

Rarity let out a deep-pitched, loud fart that lasted 16 seconds and turned very wet near the end. Sweetie Belle could feel another fart coming, and tried to fart as loud and long as Rarity could.

PPPPPPPPBBBBBBBBRRRRRRRRRRRRT!

"I think I just soiled myself," said Sweetie Belle, blushing a little, as her pants and underwear felt warmer and dirtier, and the mud around her started to smell more.

"I certainly did," said Rarity. "At LEAST for the last six seconds of my last fart. It's nothing to be ashamed of. I think it's really fun." She giggled, squirming around a bit, and Sweetie Belle did the same.

PPPPPPPBBBBBBBBLLLLLLLLTTTTTTTTTHHH!

Rarity squealed as she released a very wet brown mess into her already heavily soiled shorts for 12 seconds. Sweetie Belle giggled and got onto all fours, farting again and then slapping her butt. This time her tush felt a little more squishy against her hand, but she didn't mind.

They continued having fun for about an hour, before they got tired.

"Shall I use the crystal to clean you up?" said Rarity.

"No, just hose me down and we'll take a bath and shower," said Sweetie Belle.

"I always do it that way too."

After they sprayed each other with the hose, they took a bubblebath, often farting in the tub, and peeing at once point each. Then they showered.

"I just realized, I don't have anything to change into," said Sweetie Belle once she was out of the shower, soaking wet and with a brown stain on the butt of her pants.

"There's no shortage of outfits here," said Rarity, who had a bigger brown stain on the butt of her shorts, which still smelled bad. "You can borrow one of mine while yours in the wash. And don't worry about the stain on your pants. My outfit has cleaned up from far worse."