Demon Law

Bill hovered in the middle of the room, eye frowning at Dipper.
"Kid, you’re reeeeeeeally starting to bug me, you know that? I’ve got important stuff to go do. Shoo. Get outta here."
"Not this time," Dipper shouted fiercely at the triangle-man. "Not after all you’ve done. I’m not letting you get away. You’re coming with me."
"Kid, what happened? Did puberty mess up your whole brain? I’M A FREAKIN’ DEMON. See?" Blue flames shot out from Bill, engulfing the room before quickly vanishing. A faint aroma of singed flesh hung in the air. "What POSSIBLE way do you have to reel ME in?"
Dipper reached into his bag and pulled out the journal, flipping it open to a marked page. Bill rolled his eye. “Pfft, books. Who reads books anymore anyways? Nerds. You a nerd, kid? HAHAHA! Oh, right. It’s you.”
"Ehjrqh, irxo ghprq," Dipper chanted, ignoring him. "Ehjrqh dqg uhwxuq wr wkh khoo zkhuh brx ehorqj-"
Panic shot across the triangle-man’s face as he realized what was happening. Beams of light began shooting off of him, as brick by brick imploded into oblivion. As the last brick vanished, Bill’s eye rolled around violently before snapping shut and falling to the floor with a clunk.
"Yes!" Dipper shouted at the eye, throwing the journal on the ground. "YESSSSSSSS IN YOUR FACE! THAT’S WHAT YOU GET WHEN YOU MESS WITH ONE OF THE PINES TWINS! Oh man, Mabel’s gonna be so stoked-"
"JUST KIDDING!" Bill yelled as he popped back into existance right behind Dipper. "That was close, though." Dipper screamed girlishly as he scrambled back towards the journal. "OH NO YOU DON’T, KID!" Bill shouted, hitting Dipper in the chest with a fireball. Dipper flew across the room, smashing against the wall. "It’s time for me to take you out of the equation."
"Please - don’t kill me," Dipper wheezed, blood trickling from the corner of his mouth.
"KILL you?" Bill responded, laughing maniacally. "That’s so BORING! Nah, I’ve got something much better in mind. See, I know a certain secret that Shooting Star is keeping from Pine Tree-"
"Mabel? No, keep her out of this. This is between you and me," Dipper yelled angrily.
"Eh. It’s more fun this way. Trust me," Bill replied, winking. Suddenly, Dipper’s consciousness was split from his body as Bill ripped away control. "Kid, remember that agreement you signed with me when you were twelve? Because you were so STUPID, I can still technically use you as my puppet. That thing’s still legal under demon law, y’know.”
Dipper flew toward him, fist outstretched. “Get out! OUT!” he yelled, passing harmlessly through Bill, in the middle checking out his new body.
"Alright, alright kid, settle down. I’m just borrowing it, is all. You can have it back when I’m done, and I PROMISE no harm will come to it. Good enough?" Bill asked, flexing his new muscles. "Also, good job beefing up, kid. You must be a hit with the ladies."
"What if I don’t go with you?" Dipper asked, frowning suspiciously.
"Well, you’ll have no idea what I’m going to do while I’m in your body. And besides, then you’ll just be trapped in the mindscape forever. So it’s basically like you don’t have any choice. OH WAIT! It’s exactly like that. C’mon, kid. We’re going home."

-x-

Dipper floated nervously behind Bill as they made their way back to the Mystery Shack. Bill opened the door and walked inside, making his way towards Mabel’s room.
"Hey now, what are you doing?" Dipper asked. Bill banged on the door.
"Sister - I mean, Mabel - it is I, Dipper! Your brother! May I enter this room?" Bill asked.
"Uh, sure, Dipster. Why do you sound so weird?" Mabel opened the door, groggily. She was still in her pajamas. Without a word, Bill leaned in and kissed her.

"What the hell, man?" Dipper shouted.

"What the hell, man?" Mabel asked.

"Mabel, I read your journal," Bill grinned. "You really need to hide it better next time." Mabel’s face shot beet red. She tried to slam the door, but Bill grabbed it. "Hey! Hey. It’s okay. I’m glad it’s not just me," the demon replied soothingly.

Mabel’s eyes grew as big as saucers. “W-what?”

"I feel the same way about you." He kissed her again, but this time, she responded. Mabel grabbed Bipper and pulled him close, as she started to cry.

"Oh Dipper, I was so scared!" Mabel sobbed. "I felt so helpless and angry about my feelings for you!"

"WHAT. WHAT THE ACTUAL FUCK?" Dipper yelled, waving his arms. "HELLO?"

Mabel and Bipper kissed more passionately, each one clumsily trying to take control. Mabel fumbled for the door, locking it. Dipper floated through behind them, as Mabel roughly pulled Bipper onto her bed. Bipper kissed her roughly on the neck, as Mabel let out a soft moan. She ripped off her pajama top, breasts spilling out. Dipper was repulsed.

"THAT’S MY SISTER! HEY! KNOCK IT OFF!" he yelled despairingly at the two of them. He received no response.

Bipper grabbed her breasts, playfully rubbing her nipples. Mabel moaned again, as Bipper slowly slid his hand down her front, into her panties. Dipper flew towards the door, but he became frozen in place

"Oh no. OHNO. Bill, c’mon man, this is SO FAR over the line. You’ve got to stop." Dipper yelled. No response. "For fuck’s sake, at least let me leave. Don’t make me watch this, please." Dipper tried everything in his power to move, but he was frozen in place. He tried to yell again, but his voice was gone. He was completely helpless.

Mabel unbuckled Bipper’s belt, ripping down his jeans. Reaching into his boxers, she drew out his stiff cock, staring at it momentarily. Cautiously, she began stroking it, rubbing it slowly from top to bottom, feeling the heat emanating from it. Bipper shuddered, and Mabel giggled.They kissed again, hungrily, before Mabel bent down and began clumsily sucking his cock.

"Oh yeah, sis," Bipper groaned, "Just like that." He reached back into her panties, rubbing his fingers over her wet pussy before finding her clit. Mabel stopped, panting heavily, and ripped off her bottoms.

"Fuck me." she said. It wasn’t a question.

Mabel helped guide Bipper’s cock into her entrance, feeling it slide in, inch by inch. “Oh god, you’re so BIG!”

"And you’re so TIGHT," Bipper groaned, slowly thrusting his hips back and forth, feeling Mabel’s pussy engulf his member. Bipper slowly picked up the pace, until finding a comfortable speed.

"Oh my god, oh my god, oh my god, oh my god," Mabel mumbled, barely able to form words, "I can’t believe this is happening."

"And I can’t believe how wrong I was! Tuns out that pleasure is LOTS better than pain!” Bipper replied, breathlessly. Mabel slapped his ass in response. “Harder,” Bipper grunted. Mabel obliged.”AHHH OKAY THERE WE GO.” Bipper kicked it into overdrive, caressing Mabel’s breasts as he pounded her roughly.

Mabel mumbled something again, but Bipper couldn’t hear her. “What?” he asked.

"mmmmmm cum in me CUM IN ME" Mabel finally managed to say, panting heavily.

With a grin and a groan, Bipper did. Mabel lay panting on the bed, but Bipper’s erection had not subsided. “Hey! Still hard!” He turned and locked eyes with Dipper. “Like magic!” He turned back to Mabel. “Ready for a round two?”

-x-

The sun was rising, and Mabel lay sleeping peacefully on her bed. Bipper got up and stretched. Dipper still floated immovably in the middle of the room. “Good morning, good morning, good morning!” Bipper said, grinning. Without bothering to put on clothes, he dragged the frozen Dipper out into the hallway and shut the door.

"That sure was fun, eh kid?" Bill asked. "And look, I wasn’t lying! Your body is fine. Maybe a little dehydrated, but fine.Thanks for letting me use it! Take good care of it. You never know when I might be stopping back again! Oh, but one last thing: brx ixfnhg brxu vlvwhu. Bye now!"

And then Bill was gone, and Dipper was standing naked in the hallway.