I don't think she will change just don't let her be a horrible mother. It's a real bad situation in but you gotta ride it out and see what happens. You have got this though and do what you think is best brother.

A conversation would be the best option but that doesn't seem like one at the moment, so just worry about the child for now and see if she genuinely changes, if she becomes worse then I guess you assume custody of the child, which is something I really don't want to suggest but if it is really necessary do it.

I wish you all the best.