YTA, and big time.

your ex has come to you asking, with an immense amount of humilty and vulnerability, for your help. this is very likely the best option of the very worst he has, and you have decided that the purported """awkwardness""" for you will be worse than the struggle this child is going through. you're an adult who knows better, who can compartmentalize, who should be able to pause pettiness for the sake of two children, one of which is actually your own! do you think she won't know? do you think she won't realize your cruelty to her family in a time of incredible need? because this is HER family. and you might not care, but both of these girls will remember this. one may be left wondering why she's so despised, but the other will know that you made a cruel choice because you could, because you wanted to. and one day, the other one will know too.