He didn't say what type of sister, did he? There are a many possibilities and since I happen to know this sister I can say for certain that it is not the type you are thinking. Granted, his name suggests alot; but that is just to throw you off.
Alright alright alright. I don't know his sister. But I would think that if a dude has a crush on his sister, then she has got to be hot. And since this sister probably aint goin for the incest thing, I consider her fair game (for myself).