I fucked my 2nd wife's sister, and the wife knew about it, encouraged it, and participated in it!
The Sis had a bit of a crush on me, and was always flirting and talking suggestively if we wee a party or gathering together.
I tried to play it off, and not encourage her, and everybody would laugh about it.
Sis asked my wife about the possibilities of a 3 way or just letting us have some time alone, wife asked me about it, and I told her I didn't have a problem with it, but I didn't want to do anything underhanded or behind her back.
From that point on, they looked for any opportunity to make it happen. There were a couple times when it looked like everything was falling into place, but never materialized.
Sis happened to drop by our apartment one evening on her way home from work. And all of a sudden, it was going to happen right then and there.
We all went to the bedroom, got naked, and started foreplay. I was playing with my wife while Sis was playing with and sucking my cock until she was ready to sit on it. The wife was sitting on my face, and facing so she could watch Sis take my cock, and was encouraging her ("fuck him!", etc.).
After watching this for a bit, my wife decided to leave and leave us alone, and I fucked her silly! I was young then, had good stamina and stayed hard for a long time, and we used many positions. I never did cum, but got her to orgasm a couple of times.
Unfortunately, that was our only time. However, family gatherings seemed to be more enjoyable after that.