From what I remember, the MC lives with his mom and two sisters, I can't remember if they are step-family full blooded family. I can't remember the reasoning, but he manages to hypnotize the mom and makes her think having sex with him is normal but hides it from the sisters. Eventually one of the sisters, one that doesn't like him, finds out and confronts him before I think getting hypnotized as well. It has multiple parts and I can't find it for the life of me.