Great photograph, well composed, cool background, and the subject is very pretty. However, can you assure all of us that when this photo was taken when your sister was at east 18 years old? Not to quote Tantor, but it seems questionable to me. I don't mean this as a slight, but it is very important that this is the case. If so, I'm sorry for bothering you.
Your sister is an excellent model, has a beautiful figure, and is a good sport to particiapte in this shot. It's amazng - at the same time sexy and modest, warm but cold, soft but a little intimidating. I love it.