For some time i been fantasizing about my wife's sister, she doesn't show any interest on me or anything but for some reason she really really turns me on, til the point where I start shaking of horniness just by being around her also my excitement is so high that it doesn't go away after I masturbate. I don't want to ruin my relationship but i also don't know how to deal with these uncomfortable feelings that don't go away . Is there anything i can do to stop this?

After giving some thoughts on this i realized that what gets me on is the taboo, the idea of doing something I'm not supposed to do, this is probably because of my first sexual experience but i don't like it. This whole thing has been making me feel really bad about myself.