It's not your fault:
Mabel Pines believed herself to be a good person, that she brought nothing but joy to the world with her optimism and creativity. She always had confidence in her skills and often ignored the idea that her plans might have just made things worse. This thought process was challenged however, by a certain triangle that held her and her twin in a death grip.
"Hey, you know what? I think I'm gonna kill one of them just for the heck of it!"
Mabel's mind went into a panic! While she was obviously afraid for her own life, she was even more so for her brother. (He's done nothing wrong! He doesn't deserve to die, I'm the one that did this!) She thought to herself. She just couldn't stand the idea of watching her brother die, so she made a decision. She knew that Dipper and her Grunkles would never approve of this, but that didn't matter. This was her mess, and it was her price to pay.
"Eeeny, meeny, miney-
"STOP!"
Everyone went silent as Dipper stared at his twin. "Mabel, what're you doing?"
Mabel took a deep breath, trying to control her fear. "Please, don't take my bro's life. He's done nothing wrong. Just take mine instead."
"Mabel!" Her family yelled. "Mabel, don't you dare start pulling this sacrifice thing with me, we can get out of this!" Dipper exclaimed, trying once again to push himself out of the demon's grip.
The demon was silent at this expression of self sacrifice, that the girl was willing to throw away what little of short, human lifespan she had. Just so she could save her family. This silence though, was immediately overtaken by a maniacal laugh.
"Oh, so that's the game you're playing, shooting star? Well, well, well, what a surprise! The one who always threw others under the rug for her own benefit is going to make a huge, dramatic sacrifice to save her family! Y'know, that's actually pretty funny. You trying to redeem yourself that is. You are the one who let me start Weirdmageddon after all."
Mabel's eyes flashed with guilt, looking to the side to conceal her pain.
"Mabel, what's he talking about?" Dipper asked.
This reaction seemed to bring even more Joy to the megalomaniacal demon. "Oh yeah, Pinetree. Weirdmageddon was a team effort, couldn't have done it without her! Say, Shooting Star, why don't you tell everyone how you helped me start my little game of chaos and destruction?"
She refused to talk, shrinking into her sweater to avoid the looks that were placed upon her.
"Not going to talk, eh? Well, this oughta get a rise out of you?" The demon began to squeeze the boy in his hands. Dipper began screaming in pain, feeling his spine about to break in Bill's grip.
"Dipper! Stop, stop it! Don't do this to him!" Mabel yelled, tears freely falling from her eyes now. It was painful for her to watch her brother suffer like this.
"You want me to stop? I'll be happy to oblige once you start tell everyone how you helped me. You don't start talking, I'll snap Pine Tree in two!"
Mabel couldn't take it anymore. "IT WAS MY FAULT! I STARTED THIS!" She yelled, her voice cracking. "I-I'm the one who gave the rift to Bill. After I ran away from you guys, he came to me disguised as Blendin. He promised me more Summer if I gave him it. I'm sorry, I was selfish and I was only thinking of myself again. I'm sorry…" she tearfully spoke out, crying as tears freely fell down her cheeks.
Dipper, instead of looking furious at his sister, simply looked on in concern. "Mabel…"
Bill interrupted the quietness as he began to chuckle. "Well, that wasn't so hard, was it? Now then," Bill suddenly pulled out a megaphone made of energy, placing it in front of the young girl. "how about you tell the rest of the town your story? Do it, or Pine Tree dies."
Mabel sniffed, trying hard to speak with the lump in her throat. "I-it was my fault. I gave Bill the rift, Weirdmageddon was my fault. I'm sorry everyone."
Her voice carried throughout the town, everyone was able to hear her confession.
The demon then placed the sibling down in a cage, eagerly waiting for the events to play out. "Now that we've got that out of the way, I'll let you guys have one final goodbye before your grand sacrifice, Shooting Star. Don't worry, once the barrier is broken, I'll be sure to tell the whole world how you helped me."
"Mabel, please, you don't have to do this! Let me sacrifice myself, you don't deserve this!" Dipper yelled, rushing over to embrace his twin in a tight hug.
Mabel gently returned the hug, a sad smile tugging at her lips. "Bro, come on. You know I can't let you do that... *sniff* Didn't you hear? I'm the one that started this, I should be the one to pay for it. Besides, you've already done enough for me. Every single time I've wanted something, you would give up your own happiness, just so I could get what I wanted. I'm sorry. I know I've been a total poop head of an older sister. So please, just let me make it up to you."
Before Dipper could argue back against his twin, Bill suddenly picked them back up. They were trapped in his death grip once again. Dipper looked on in panic, as his sister looked to have accepted her fate, with her eyes closed and looking down.
"Alright, eeny, meeny, miney, yo-"
"Wait!" "Ford" yelled.
—
After the events of Weirdmageddon, Mabel sat quietly in her room, petting her pig who had lied down on her lap. Even though she was happy to help get her Grunkle's memory back, she was now incredibly afraid and guilty to even face her family after her confession. She didn't want to meet the disappointed looks, that she was certain were there. She jumped a little at the sound of the bedroom door opening.
"Mabel, are you okay?" Dipper asked, slowly entering the room.
Mabel let out a small sigh, looking down at the floor. "Y-yeah. I'm fine…"
The room was accompanied by silence, until Dipper broke it with the words Mabel never thought she would hear.
"It's not your fault, you know." Dipper said.
The older twin felt herself begin to choke up, tears now falling down her face. She embraced her sibling in a tight hug, crying onto his shoulder. "Bro, come on, we know that isn't true. You heard, I gave him the rift. I don't think you can spell it out any clearer."
Dipper, instead of letting her go, only chose to tightened the hug and argue with her. "Mabel, we both know you never would've even ran away if I had just considered your feelings before accepting that apprenticeship. I got caught up in the moment and made a selfish decision. It's my fault."
"Bro, don't you dare-"
"Yes, I dare."
Mabel let out a small groan. However, in that same moment, she began to chuckle a little. "You're really selfish, you know. Whenever I try to take the blame for something, you always make it sound like it's your fault. I never even get the chance to apologize. *sniff* And here you are, making me cry. When did you suddenly become the older twin who makes the sacrifices, you big dummy?"
The two shared a small laugh, just enjoying each other's company as they hugged it out.
"Well, how about we just blame Bill then? To be honest, he's the one who actually did the Weirdmageddon thing. Makes it a lot easier on us, too." Dipper asked, removing himself from the embrace.
"Yeah, that sound good." Mabel said.
Dipper then put his hand on Mabel's shoulder, a sad look forming in his eyes. "Please promise one thing though, Mable. Never, and I mean never, try and sacrifice yourself again. You nearly gave me a heart attack! I could never stand losing you. If you ever died, I'd make sure to follow as quickly as possible. If we go down, we go down together, okay?"
Mabel gave a quick laugh, scratching the back of her head. "Okay, bro. Promise. Pines stick together until the end, right?" She said, extending her fist.
"Right." The two fist bumped, each letting out a deep breath.
"Well, I think it's time we got back to the living room. Grunkle Stan's making pancakes." Dipper said, standing up and opening the door. "Come on, Mabes!"
"Got it bro, coming!" She exclaimed, racing her brother to the kitchen table with excitement. After all of the silence and pain she had to go through, she was happy to finally feel that huge weight being lifted off of her shoulders.