My mom's at the end of her rope?

Im sorry your in such a hard situation, and i really dont know what you can do besides have a nice respectful long talk with your mom about the fact that she has to do whats best for your sister even in this hard situation and after all shes been through. Otherwise you might have to wait for her to go to kindergarten. Maybe a neighbor, grandmother can watch her? It would have a very bad affect on your sister if she where g-d forbid abused by your mother, please do what you can although it may be hard.