Another favorite AU: TIMESTUCK, where Mabel gets stuck in or and Stan picks her off the streets. I have read the same story 5 different ways and I just love it so much. Dine and dash pancakes, Mabel and Stan bonding in no.
It would be great since Stan is on Mabel's side most of the time (toughening Dipper up) if his brother would favor Dipper rather than Mabel and express this through, I don't know, singing. By Sammymations & Art
Since it can run in families, I had a headcanon that when Mabel grows up she also has twins, and every year she takes them back to Gravity Falls to see the old crew.Oh, and she still wears her hilarious sweaters ^U^ Original sketch available here.