First let me welcome you to our forum. Thanks for coming.
I'm sorry to hear about your traumatic childhood, it must have been awful. I don't know what would or wouldn't be a good argument in court but I can tell you this, bipolar untreated is a very nasty beast and can turn people on their nearest and dearest. The "highs" aren't always "high" in a good sense, but the body is full of energy, and sometimes that energy is dark and angry. The lows are also dark and life feels impossible. I'm sure you've seen both states. In particular, the more far gone the mania is, the more likely the person is to make terrible decisions and act irresponsibly. That's one of the key symptoms of mania.
That doesn't make what she did right. At some point we are responsible, despite the disorder, for behaving like adults and taking care of our responsibilities. For some people that is impossible and they need to be hospitalized. Otherwise untreated bipolar without any checks on responsible behavior can end up looking like your childhood for example: life with a detached, violent parent. I don't know the story, so I'm not sure why she wasn't medicated or why no one insisted she get help or helped you, but she was fighting with a very real and dark illness -- that is true. I feel bad for her in that regard. On the other hand, we all need to get help, any one with bipolar needs help getting treated and managing the illness. Resisting help and taking her frustrations out on a little child is horrible. I'm sorry for you.
I'm sure it is hard to be around her. It probably will be for some time. What I would suggest is that you get yourself a therapist, someone who can talk through all these experiences and pain with you and help you come to terms with them and get beyond them. It's a long road, but it's very worthwhile. You'll feel better when you can talk about it.
Co-Moderator, Bipolar Forum