My Wife and SIL do it
I knew my wife had a sexual interest in our SIL, but was surprised when I learned he was actually coming over to the house when I was at work to do her. I happened to drive by the house one morning and noticed his car there so I went in and that's when I heard the moans, headboard hitting the wall and dirty talk coming from our 22 y/o SIL as he was banging her. She was begging him for it and he was telling her what a dirty little slut she is. I froze in my tracks watching this and once I heard him tell her he was going to cum, I actually lost control and unloaded in my dress pants. Feeling humiliating for cumming from watching such a sight, I quietly left.
He knows I watched now and he likes it when I'm there on my knees beside the bed watching him give my wife the kind of sex she wants.