I cheated on my wife with her sister

My wife and I have been married for 7 years now. Even though I love her, my wife's younger sister is just too seducing. She visited us and when my wife wasn't there, she dressed up in a bikini. Then I automatically fucked her. It was great sex but I knew it was wrong. I really regret it. I never fucked her again. But something inside of me wants to and doesn't want to. If this normal?

By Anonymous