How to help my mom help herself....?

My mom is bipolar and completely dependent on my dad for support. There are seven children and two of them still minors. My dad is my moms trigger to have a manic episode. He treats her badly and currently is having a blatant affair and she is putting up with it. It is a matter of time before he drops our family to be with this other woman. I could care less whether or not he goes. The problem is I can't get my mom to get a plan of action together for when he does. I can't afford to care for her and my two youngest sisters and neither can my other sisters. I have no idea how to shake some sense into her and make her understand she can't ignore this away.