Is it normal to think about having sex with a close relative?

When I was a teenager, my dad was gone quite a lot, so I ended up sleeping with my mom and even taking baths and showers with her. She said she was only letting me have sex with her to take care of my "needs", that she herself did not desire it or derive any pleasure therefrom. (For the record, I think she was lying. I think she was just as desirous of the rlationship as I was.)

My wife was the sexual victim of 3 of her 5 brothers for years and one of them ended up getting her pregnant when she was 20, which led to us getting married.

Who else has at least thought of the possibility of having a sexual relationship with a close relative, especially in the nuclear family? Or had erotic dreams about that, or fantasies?