I don't know what to do with my mom.?

I am 20 years old and I live with my mom, her husband and my 3 younger sisters. I've known for a long time that my mother was mentally unstable and over the past few months, it's been getting worse to the point where I don't know what to do. She is "right" no matter what and refuses to listen to us. Her husband is the scum of the earth and I always ask why is he still her. He treats her badly and has done things that any person would hate him for. He's been with her for about 4 years and my sisters and I hate him so much. Ever since they got together, he has done nothing but use her and manipulate her but she doesn't see that. Yesterday she blew up about my boyfriend who I have been with for 3 and a half years and is telling me I can't see him and that if he comes over, she'll call the cops :( Ever since I can remember, I've been helping raise my younger sisters and I'm scared for them. Yes, she provides a roof and works to take care of us but that doesn't mean she can throw temper tantrums and call us horrible things for no reason. She has a "you're either with me or against me" type of mentality and it's ruining our lives, We don't have any family here and I don't know what to do. My boyfriend has been frustrated with her for a long time because of how she has treated me at various times and he knows its not fair to me. I wish I could leave and take care of my sisters since they are not too young (13,16,17) but I'm stuck. She called us tratitors because no one agrees with the way she behaves. She has tried to commit suicide and continues to threaten us if we don't agree with her. This isn't fair to me or my sisters. I work 2 jobs and am trying to get into nursing school. My sisters and I are scared and don't want to stay with her but we have no one to go to. Help please?