I've been very fortunate that I haven't been to many funerals in my life and of the ones I've been to, the departed did not have small children. But hypothetically? Yeah, I could certainly picture two close friends at that age holding hands at a funeral. The scene doesn't really scream "siblings" to me, but it's certainly an indication that they've been close for a long time.
The scene in the bedroom? That scene I
can speak from experience because my best friend growing up was a girl and we definitely did start... um... exploring our bodies together in high school

. All of that same excited awkwardness and sexual naivety was there and we talked ourselves into doing it under similarly ridiculous pretenses... absolutely any reason other than admitting we were horny teenagers attracted to each other. So yeah, that scene definitely feels way more authentic as "childhood friends". I really can't imagine siblings doing that in reality. Even in a fictional sense, I agree that siblings in that situation would probably be a quite a bit more hesitant.