In this Gravity Falls AU, Mabel and Dipper have an older sister, y/n. She is 3 years older than the twins, making her 15 at the beginning. Will the events of this summer make y/n's and the niblings close or make them drift apart? Bill Cipher × Fem! Reader Really slow burn, stick around for the story! I so not own Gravity Falls or the characters......except y/n..... I own her.....