I'm so sorry this happened to you and your sister, OP. It's awful when you are betrayed by the one person in this world you should be able to trust-- your mother. Every interaction you have with her, she will win, because she has the power. You are wise not to do anything too aggressive, so you won't have big problems while you wait out the two years until you are old enough to move out on your own.

But there is something you can do, and it has devastating power. Ignore your mother, stepfather and brother. Never speak to them (unless it is absolutely necessary.) And... don't look at them. Do this especially with your mother, and teach your sister to do it, too. When you come into a room, only look at your sister. Only talk to her-- and talk normally and act naturally with her. But don't give any attention to your mother.

She will soon feel your power. Only speak to her when it is necessary, like, "You need to sign this paper for school." But when you speak to her, look a little off to the side. Do not make eye contact. If she starts talking to you, just sit there. Don't respond. She may start yelling (out of her increasing frustration.) Don't react. As much as humanly possible, act like she doesn't exist.

Wait til you see the absolute power of this tactic. It's devastating.