Sex with siblings watching?

Has anyone had sex while a sibling was watching? Or even participated in the act? If yes, what was their reaction? To clarify: I do NOT mean sex with siblings or incest.

I was in my teens when with my brother and his girlfriend (both in their very early 20s) went skiing in the mountains. We rented a small bedroom with 2 beds, I took one and they took the other. In the early morning hours, funny noises woke me up and realized they were having a good time but kept pretending I was asleep, too scared to react. Next morning, a repeat session. Quite a few years later, with my then girlfriend we visited her sister and hubby. After a long discussion about relationships and sex and after a few more drinks, a sexy tape was playing (not sure how this started). Hazy memories, but I remember well we had sex on the couch, just straight sex, each one with their partner like couples, never swapped partners. I remember there were times the sisters were holding hands. Next morning, I woke up with the guy's genitals just inches from my face. Again, no recollection of what transpired. We were all naked and with a mild hangover. We spent most of the morning naked and just being lazy. Unfortunately, never repeated the experience.