Thin Walls Reveal Consanguinamory

From Full Marriage Equality:

I live in an apartment next door to nice guy who's going to college. He has lived there for almost a year. Friendly guy who is single. He doesn't appear to have steady GF, but he does bring home girls occasionally and has sex. The walls are pretty thin, and I can hear them having sex. One girl that has been there several times has been his sister, and every time she stays the weekend, its clear they have sex several times. I've met her and talked to her down at the pool, and they seem like an affectionate couple when are out in public, but he always introduces her to everyone as his sister.

Its really none of my business so I have not said anything to him about hearing them have sex through the wall, but sometimes I wonder if I should just so he knows. I've not said anything because hearing others have sex does not bother me, and I don't want make him feel he needs to hold back.

[...] I don't know if they are blood related. She did come with his parents to visit once. They kinda look a like a little, but I don't know if that just me looking for anything that's similar, like they laugh the same way, and they both have green eyes. Sometimes I think they act like brother-sister, then other times it seems like they act like couple.

[...] They even have the same last name. He just doesn't know my sister and I can hear them having sex when she spends the weekend. We had not seen her for a while, but she spent last weekend with him, and well, here was some intense bedroom activity occurring while she was here.

[...] I try not to be judgemental about such choices if is a mutual choice\attraction. When I was 14 I had sexual encounter with a cousin who was 16. I was already sexual active, but he had never had sex before. One evening we were alone together and began engaging in foreplay that led to us having sex.

[...] I was willing to have sex sooner than [14], but he was very hesitant and prefered we limit our encounters to oral sex and sleeping the night together. Even though we are cousins, he is always seemed like a brother to my sisters and I, so while technically we were kissing cousins, it was more like of a brother-sister relationship. When we did start having sex, it was pretty intense, and it does remind me of what I hear through the wall when my neighbor has sex with his sister.