The reason why your mom wont leave is becuase at the age she is at, its hard to be independent when she was dependent so long. She just dont know. Its scary for her out there. Who is she going to lean on? Even though your dad was/is bad just having him there, knowing he is there gives her reassurance that she can handle things that comes her way. Plus, at her age she knows its alot harder to find another suitor. Its a horrible feelng when she knows she will be out there in the world alone. Yes shes got you kids but its still different.
I dont really know what to tell you. And you know your parents wont listen to you either, they probably still view you like little kids unfortunatley thats how all parents view thier kids no matter how old they are.