I assume you want it banned because of secondary smoking, so justify it in the context of children. Do you think the overall negative impact of having a smoker for a parent is greater than the negative impact of having a parent who had sex with you since childhood?

Yes. If that parent is nurturing and fosters positive traits, I see no reason for harm to be part of this calculation. Sexual interactions (not necessarily penetrative sex - that's quite rare in pre-pubertal individuals who are victims of sexual abuse, more common in ephebes) can serve as a powerful mechanism of pair bonding that can act to strengthen familial ties and improve trust between individuals - as long as you don't have a culture that condemns you as a victim with no agency, and your parents as rapists who are going to jail.