I just want to say that I was in a very, very similar situation a few years ago, and you have my sympathies.
My situation resolved itself, as my mother's boyfriend pressured her to get my sister out (he doesn't live with her, but he saw, rightly, that my sister was making enough money to support herself and at the ripe old age of 26, was more than ready to live on her own), so she did. My sister stopped speaking to my mother shortly after that, for reasons that she has yet to articulate in a coherent manner to me, and I think it's because she's resentful that the gravy train came to an end.
Before and immediately after my sister's moving out, my mother and I had a very strained rel@tionship, almost to the point of permanent estrangement. She would often throw me out of her house when my sister started fights with me, because "we" couldn't get along, and she couldn't throw my sister out, because she lived there. I was very resentful. Of course, my sister took this as license to start fights, because she had nothing to lose from the situation and it would always work out in her favor (i.e., me getting thrown out)
Now, my mom and I are better. I no longer speak to my sister anymore, either, after she told me that it was my fault that she had to check herself into a hospital for depression. I wish her well, but I know that if I let her back into my life, she'll just resume using me as a human toilet again, and I just don't gain anything from that sort of interaction.