Never done this within the family. However, many years ago friends of my parents invited me and my 25 year old wife over for dinner. They were in their fifties. My wife had had a some experience of having sex with older married men before we were married, but I was a bit sheltered. We were all quite drunk and we spent a few hours swapping later on in the night in seperate rooms, but finished up together on the lounge room carpet with the ladies missionary.