Is it weird for a guy to date a woman who looks like his sister?

Well, if you look anything like your bother, not being a jerk saying this, it's likely he doesn't see the resemblance. There are studies that show we are attracted to a certain look and it's usually dealing with our own appearances or based on those around us the most. It's not even a conscious thing, but there is something about the symmetry of the face that appeals to the one looking. So, though it might be a bit weird, there is a science behind it being something natural and not as weird as it may seem.