Going away to start a life for yourself is clearly the right thing to do, as many others have commented.
I feel I must add that the situation with your siblings is very concerning and I think you ought to consider intervention by professionals. By professionals, I’m talking about child protective services, school counselors, therapists, and the like, including the police if your mother is ever physically abusive.
Not only could the intervention of these professionals help to avoid further abuse by your mother, but they could also build the case on why your saint of an aunt should be supported in her bid for guardianship… not to mention that it sounds like you and your siblings could use the advice and expertise of such people to help get your lives straightened out after dealing with such an abusive parent.
Good luck and it sounds like you are starting off in the right direction.