Wife's Sister

I have always had a thing for my wife's younger sister ever since I was first introduced to the family. Never acted on it but the feeling has always been there. It's got to a point now that I can only cum now if I'm thinking about her. I watch a lot of porn and I'm bisexual so men turn me on as well and I masturbate to it every day but cannot cum until I think of my wife's sister. As soon as I do I usually cum instantly. I really want her so badly . She is not with anyone at the moment but obviously I'm still with my wife but would give up everything to be with her sister. I just can't help the feelings I have for her.