I would leave this man immediately. I cannot think of a single situation in which sexual relations which one's family member is appropriate. His mother definitely has serious issues but he was a full-grown adult when this began and ought to be able to control his impulses. Personally I cannot live with someone like this but then and again I am not in your shoes and the decision ultimately rests with you. If you believe that he wants to change and seek help then perhaps you may be willing to stay with him.