I've got a few dozen good stories.
One of my favorites was about 6-7 years ago i was dating this Asian broad who had a smoking hot sister. We went on a vacation trip to San Diego as couples, my GF's sister brought her douchbag hipster boyfriend.
First night we are there everyone gets super drunk, GF sister's BF passes out on the couch early. GF sister passes out in a recliner. Around 1 am the BF leaves to go sleep in his room, so i end up laying on the couch watching TV. I'm half asleep when the sister comes over, sits next to me and starts playing with my dick. After she starts blowing me for a few minutes she realizes i'm not her BF.

Freaks out and runs out of the living room.
We never spoke about it, but it was a weird vacation after that.
Another one, i was dating this broad from El Salvador. One of her best friends wanted me badly, always flirting with me and giving me eyes. Once i broke up with the ES chick a week later i start banging her best friend. She was a freak, loved to dress up and play with toys.
My ex finds out and they get into a huge fight, both super upset at each other but not me.
I end up banging both of them "separately" for about 3 months. They were competing for me. Both knew i was banging the other, it was one of the most amazing moments of my life.