I live with my girlfriend, and last year one of her college friends was living with us for about 6 months, i'll call her S. So after a few months staying at our place she tells us that her dad will be coming to visit and asks if he can stay at our apartment for a few nights, we said sure no problem. When he arrives we realise he's an extremely rowdy country guy, who has an incredible drinking problem and talks louder than anyone could possibly tolerate.

So the night goes on and we're drinking with him, but he's much more drunk than the rest of us and is making inappropriate comments and jokes which leads to S being pissed off enough to say she's going to bed early and leave to her room. My girlfriend wisely decided to do the same, since she had work the next morning.

So I'm stuck awkwardly with S's drunk dad while we smoke cigarettes and drink beer on the balcony, where he proceeds to try and have a manly chat about sex. He went on to tell me graphic details about what the sex was like with S's mum, all their kinks and fetishes etc.

Then he let it slip that they had another kid before S, and that she has no idea that she has an older brother somewhere. After telling me he went red instantly and told me he didn't mean to say that, and I could never tell her.

TL;DR Housemate's drunk dad told me that housemate has an older brother that she has never been told about.