Very sweet!!! I love your Disney-esque drawing style; it suits "Gravity Falls" well. The only thing I'd fix is the anatomy: their heads seem a bit too big---unless that's what you're going for as part of your art style. But other than that, I love it !
Plus I love "Gravity Falls"
If you're interested, I got some GF-related stuff too (feedback welcome!) : [link] [link]
It looks like Mabel gave Dipper a glomp tackle, either for something he did or said that made her happy, and better yet he's just pleased that she's so elated. The style is pretty darned good: clearly the Mystery Twins (it grows on you), even mixing in some other Disney styles (I think...). It's a lovely and fun little picture.