Is it normal that I jerk off thinking of my sister's boobs?

You're fantasizing about incest. You should ask your sister to let you cum on her boobs... that's as far as it has to go. After you explode your load on her perfect tits, you'll either stop jerking off to the fantasy (because you made it real)... or you'll want to keep doing it (because, believe me, the real thing is better than the fantasy). Either way, you'll be happy.