Is he fantasizing my sister?

Me and my boyfriend have been together for 1 year and 2 months. During this relationship in which we call “ serious and long term relationship”, we discovered all our sexual preferences and tried them all together. Within these preferences I discovered that he likes me to pis* ( I am so sorry to say that) on him and other stuff similar to this preference. It is not his top thing but he likes it and likes to see me pooping. He also knows that I have a twin sister.

One night we were talking about childhood and about funny memories, then all of a sudden he said “I would like to tell you a funny story about my little brother, I used to wake up soak and wet because he used to piss on me.” Then he laughed and suddenly asked me “has anyone did this to you make you soak when you wake up in the morning”? Did he mean my sister??? Did he want to fantasize my sister doing this?

Excuse me if this was ridiculous question, but we are living in a terribly weird world and I am an over thinker. I asked him and he denied.. please advise and what is your analysis of these information?