As I see it, you shouldn't be looking at any women (save your wife) in a provocative or suggestive way as you're MARRIED. I know you've mentioned your niece is very attractive, I can understand that. However, since you are married, you made a commitment to your wife to love her and only her (unless your marriage is an open one).

With that out of the way, perhaps you shouldn't be near your niece as she is, without knowing it, tempting you in ways that could very well jeopardize your marriage.