Swinging takes a turn

My wife and I are swingers and go to sex clubs where we can just watch or participate. Last night, we were at a club and a girl in a mask was coming on to me. My wife encouraged it and I started to fuck her with my wife watching. When I was done, she took off the mask and it was my sister. She and my wife set it up. I was so tuned on, I fucked my wife with my sister watching. On the drive home, my wife told me it was a fantasy of hers to watch incest. Now I am not sure how I feel about it.