There's a difference between accepting it as ok, and accepting the fact that she may have done her best, and it doesn't sound like she had any help, safety net, or anyone looking out for her. I'm not saying it was ok, or that you don't have the right to draw a boundary and take a breather, but the discussion of "cutting her off" because you don't approve of her behavior seems harsh and premature to me. When you say she neglected you, you only mention the friction between you and your brother, and that you had to take a job at 15 to put a lock on your door. Did she also fail to meet your basic needs, or is that the neglect that you're referring to?

Just saying, I read another comment that said you understand firsthand how crippling depression can be, but depression is a spectrum, and I very much doubt that you do understand the state she was in. It doesn't make it ok. You don't have to accept it as ok. But cutting her off entirely, based on what you described in this post? I wouldn't go there right away, for her sake and for yours. Sometimes depression turns us into people we wouldn't even recognize in our right state of mind. It doesn't mean anyone has to be ok with it, but I also don't recommend passing judgment or assuming she'll never change either, that's all. Take your time, draw your boundary, let the adults in your family do what they should have done long ago and see if they can get her help, but coming from a family with a history of truly life-altering bouts of major depression, I wouldn't write her off or close the door entirely.