Is there any way you could talk to your biological dad maybe? Does he still have a girlfriend? Can you stress to him how important it is that you need to get out of that house. This could really blow up at any time & you need to protect yourself.
Do you have a local women's center? They are a fantastic resource! Just to have someone to talk to. Maybe you don't wanto talk to your mom by yourself. They can help you w/that.
I'm thinking IMHO, that when your mom hears this news she will be saddened that you didn't come to her right away. Give your mom a chance to help you. Don't hide this. Please!