When I was 40 and my wife, Sandy, (35), and I were pretty heavily into Swinging and Swapping. Sandy would do any man.
My younger sister, also 35ish, was quite liberal too and had a new boyfriend, Ron, who was shy, but curious.
Sis wanted to Initiate her boyfriend and asked if my wife and I would swap with them. Sandy had no objection, but I had no interest in porking my sister. Sis assured me, WE would just fake it while her boyfriend used my wife. So we arranged a visit and started by playing Strip Poker.
Short version; we lost all our clothes and "bet" swapping partners. Sandy took Sis's date to another room and was soon romping and moaning with him. Sis and I sat on the couch under a blanket and listened for them to finish. I couldn't help from getting a raging b**** and Sis was fingering herself.
Sandy and Ron were taking forever. Sis noticed my B**** and said, "This Sibling Taboo is silly. Let's just do it." And she straddled me and lowered herself over my stiffy. We satisfied our needs that night, but never swapped nor spoke of it again.