It sounds like his life is very chaotic. Unfortunately children act out by behaving badly and sometimes target one person for their discontent. She seems to be of the school that negative attention is better than no attention. You can't stop her behavior right now. She has fixated her anger on you. Maybe because you are so different from her mom. Some of this anger may be towards the situation. She has a broken home now. And she focuses this anger on you. Sometimes children will see the girlfriend as the source of all her problems. You can't fix this because you are the imaginary source of the problem. The common denominator is her father. He has to fix this. Seeing someone in his church is a great idea. If this doesn't happen you can log onto the American Psychological Association and find a therapist near you. It may mean calling around for someone who does a fee sliding scale.
I think the situation is temporary since your partner has included you in this situation. I would communicate with him but let him handle it on his end.
Please press accept; this is the only way I am compensated