Cheating mom

I know this is in the wrong section but I don't know where else to put it.. My mom is cheating on my dad, for the 4th time. The man has been in and out of prison, and she knows my siblings and I hate him. She also knows if she gets caught my dad gets custody of us, and none of us want to see her. I don't know what to do.. Opinions please on how to handle this situation. (To make it worse she talks to him with myself and my siblings around!)