Grown up Dipper and Mabel

I've been dabbling with Gravity Falls fan fics of late, and I may have a new one for toi guys concerning Robbie Valentino.
I think Robbie is adopted, and here's why.
Think back to the episode "Love God" when Mabel is walking down the hall to Robbie's Room and passes the portraits on the wall.
(1) Young Robbie, Happy, Gleeful. one of the first pictures, before he found out he was adopted.
(2) Robbie, about Dipper and Mabel's age, still happy, but slightly distant. Robbie was a smart kid, just like Dipper and Mabel, with natural musical talent, Julliard level talent, and it's about this time he found...