Teenager, bi-polar ex, still in love, but how much can one take?

About us, we have been dating for 5+ years, living together for 3+. Her teenage son is controlled by his dad and he moved in without even saying goodbye. She has a mid-teen girl who is in a world of confusion. Since daddy can't control her, he slams her and cuts her down. The rest is completely blamed on my GF. The boy believes whatever his dad says and the girl just wants away from both of them. I have always had a decent relationship with the kids for being that other man, but lately, the girl hates everyone. My GF has asked me not get confrontational with the kids or her raging ex. Note, they have been bitterly divorced for 12+ years. It has taken it't toll on me to just suck it up and hold back from speaking-up/protect my GF from the attacks on her person. We are very much in love but yet we are now not the people we were when we met. Yes, I know things change. I can't imagine any circumstance how we can even get back to 50% of the way we were. We are both closing in on 50 and I don't like being bitter and surely don't want to be for the rest of my life. So to start this thread, do I continue to hold onto hope and be there for her or do I start to move on.

No, I truly can't. That is just how I am built. I cannot sit back and watch the people I love be attacked, trampled, etc. That goes for family, friends, etc. While I am not a vicious man or a bigtime fighter, that does not mean I will not have someone's back. That is part of love, friendship and family. So you can see one major part of my issue.