you were the adopted sibling of Mabel and Dipper Pines and you were staying in Gravity Falls, Oregon with them at grunkle stans place..the mystery shack

you follow them to the shack/shop to meet grunkle stan

Grunkle stan: Hey there kiddos..Mabel and dipper and youturns to you must be the their new sibling..nice to meet you kiddohe says