My wife and I were sharing a room with her sister on a trip. Late that night, the sister-in-law comes into the room, thinking we're both asleep, but I wasn't.

She then starts to change clothes, getting totally naked in the process. I don't know if she knew it at the time, but she did stay naked for a long time.

I still think about her naked to this day, and masturbate a lot to the memory of her perky breasts, and nice ass.