For those who are citing incest, I would like to remind you she inserted him into her womb after he was already mostly gestated. Genetically he has no relation to her (even within three months the only thing a fetus recieves is blood and nutrients, absolutely no genetic material). Basically, since he wasn't concieved by her, she may have given birth, but he has no genetic relationship with her.
It's like a surrogate mother. I won't argue against people who would state anyone raised having a relationship with the one who raised them is iffy, but I wouldn't call it incest.
(P.S. I am thinking more of his "siblings" than his mother, but since she IS a SUCCUBUS... ... So what? i.e. look up what a succubus is, really, and tell me this is worse... And I am referring to the original mythos.)