Long story short, my wife and I sometimes invite my sister and a friend of mine over small social/drinking get-togethers, and they will usually end up having sex in our guest bedroom. I made a detailed post about it [here](https://www.reddit.com/r/overheardsex/comments/orhumi/my_wife_and_i_overheard_my_sister_having_sex_again/) if anybody is interested to know the back story.