You can't help her. She is the only one that can help her. What you can do, is set boundaries for yourself. Such as "I will not take abuse from you. If you try to abuse me, I will XYZ" (XYZ examples would be leave; the relationship or even the situation like disengaging or walking away, leaving the house, etc). Or like "I will not engage with you if you are drinking. If you want to have a conversation with me, then you will need to do that while sober". You aren't telling her what to do but rather what you will and will not put up with in your own life. You aren't saying "you cannot drink" but rather "I will not engage with you if you are drinking". Big difference.
I would be careful to excuse abusive behaviors. PTSD/CPTSD, any other mental illness and even trauma alone does not excuse abusive behaviors. It just doesn't. It is up to the person with these illnesses and trauma to learn how to manage their own triggers, their own behaviors, and their own reactions.
Back to helping her. You can be supportive (while still having and adhering to your own boudries) but in the end, only you can help you and only she can help her.