Safety from threats by husband

Hello sir ... My father got expired 25 years back and my mother got second married 20 years back. My step father is a street hawkers. He is short tempered and I think he has some mental issue as well. In spite of this my mother has been living with him. We live separately. My mother remains sick most of the time and can't bear all this any more. Whenever my mother try to take step to get separate, my step father threaten her, that I would commit suicide and write a letter on your name. My mother get scared. Please guide me what step can we take to be safe. If he does something like this. I don't think so he has this much courage, but still what should we do for safer side. Thank u sir Waiting for your response.