The first time my brother fucked my wife was back in 2014 at his house while we were visiting. I don't even know how it started that night but one thing led to another and I ended up watching my wife and youngest brother have some very hot, very passionate sex together. Afterward, my wife said, "Wow, that was really unexpected but it was hot." My brother apologized several times saying he didn't know what came over him. Nothing else happened on that trip the following days we were with him. But, since then it seems anytime he comes to visit us or we go to visit him (about once a year) they both go at it again. It is always super passionate and erotic. It always happens very spontaneously too.
My wife, my brother, and I went camping at a local lake not long after we returned from Vietnam. We all were in one tent. My wife asked if my brother was still "cherry boy." I told her I thought he was. She asked if she could teach him to fuck, and I told her she could. They fucked four times the first night. The next day, since we were in a VERY secluded place, she went naked all day, and every time she saw him with an erection, she gave him another shot of pussy, and made sure he knew what to do to make a girl orgasm.