20 years ago my sister had to have an abortion after I got her pregnant. We were at friends place staying there for a birthday.
Lots of drinking and a lot more. I called it a night about 1am and went to bed after a good few drinks and a 10 hour drive there the day before.
I was sound out asleep when I was woken by someone getting into my bed with me. Still full of drink I almost immediately went to sleep again only to be woken again with this person pressing their ass against mine as they rubbed themselves.
Realising what was happening, I get into it and push back against them. She fumbles between her legs and strokes my cock with her wet fingers before guiding me inside her. Full of vodka and horny it didn't take long and I was blowing my load inside them before passing out properly.
The next morning, I wake up with the mother of all hangovers. Getting dressed I find a pair of goofy SpongeBob socks on the floor and it dawns on me what happened that night.
My sister was almost blackout with drink and had left the room to be sick. She was put to bed in another room but hadn't grabbed her socks before leaving the room I was in.
The realisation of what had happened hit hard. Guilt and worry but also a sense of it being a major turn on.
3 weeks later she's having a meltdown because she was late. She told out mom who was cool and helped her through it but she couldn't tell her who the father was. It may seem like a fantasy come true but I was absolutely petrified.
To this day she doesn't know that the stranger she hooked up with was her brother and it's staying that way.