"Another approach might be to say "Yes, he is family. DD is also family." You might go even farther and say "By his own, voluntary actions, he forced me to choose between members of my family, and I chose my DD. Why do you think I should choose him over her instead, given that none of this is her fault?"."
The problem here is that this is JADEing (specifically J with a dash of D). The people who would continue to harass you over such a decision don't care why you think it's justified, and so any explanation is just an extension of the discussion and an opportunity to try to change your mind. "Talk to the hand" is the only tactic that works every time, because that allows you to control the interaction. So, the best answer is to say "no" and nothing more, and then cut them off too if they can't accept it, at least until they learn to accept your decision and stop harassing you about it.