I would tell her that you do not mind being their for her, but do not want to be a part of her "smear campaign" against her family. If she is a mature adult she will understand that. If she does not then you should just cut all ties. You have your own problems I am sure, and should not have to waste all your energy on hers. Do not her vindictiveness drag you down, or make you be a part of something you do not want to be involved in. It is not high school, and that kind of drama in your life is surely draining you.