What if Mable never told dipper what happened that year? Every Year they come back to Gravity Falls. When one day Dipper and Mable come to visit at age 17 Mable finds out that the universe destroying triangle is really alive, but human? She ends up doing some 'things' and getting pregnant with his Daughter. So when she turned 18, which was only a month after she got impregnated, she moved out of state from her brother