[MF] My wife and I overheard my sister having sex and got so turned on

I’ll try to make a long story short. This originally started when my wife and I decided to have a very small get-together at our condo on New Year’s Eve 2020. We only invited two people: my sister and a friend of mine from work. Both had been COVID tested very recently, and we specifically chose them because my sister is single and lives alone, and we didn’t want her to be alone. And my friend is also single and lives alone, and he has some on-and-off issues with depression and anxiety; nothing too severe, but I feel bad for him because he’s a really nice, humble guy. He’s the shy introvert type who doesn’t have much luck or experience with women, so I thought it would be good for him to have a fun night over at our place without too many people to make him anxious.