My mom is insane!!! What can I do?!?

Okay, I'll tell my life story here real quick. My dad has a disease that makes him have headaches all the time, so he's on disability. He says he lives with his mother but he lives with me and my mom in order to raise me (but tells the government that he lives with his mother and sister). He gives all his disability money to my mom for bills except for some for gas money. My mom is insane though and I'm really scared. She always starts fights for no reason whatsoever and criticizes everything that everyone does. I can't even talk to her anymore because I'm so scared about being bullied about something I say (she's that crazy). Anyways, my dad always leaves like once a year after a fight but always comes back for no reason and makes me suffer through all this. I plead for him to leave but he won't for some reason. He drives him insane and me insane and today my dad did nothing and she just randomly yelled at him starting a fight. My little cousin fought back for him and made my mom speechless, until eventually she stormed out of the room and is in her room now with the door locked. My mom has tormented me for the longest time and made me this insecure unsocial little monster that finds life useless and tiring. I want to just fight back but never get the courage. Tonight my dad is going to this sleep place for his condition over the night and I'm afraid to be alone in the house with her. What should I do?!!