Is it normal to wank over your sisters ass?

Errm. Whether or not it's normal you probably shouldn't. Most people have fantasies of certain members of their families but do not voice them.(Usually) It is quite normal to have such thoughts. Seeing as she is a female, she has a curve your mind enjoys, and you see it often. Your instinct is more latched to her bottom than that she is your sister. But I advise not doing so.

Man if she is hot just have her see you jacking off to her and knowing white girls there are the most horny race. So 80% chance she might even let you fuck her.