Anyone can legally say anything to you. It is your choice as to whether you are going to listen. Typically, kids aren't great at taking direction from their biological parents and absolutely fail at listening to step parents unless they have a great deal of trust and respect in the relationships. That trust goes BOTH ways. I would guess that isn't happening at your house. Well, what do you want to have happen? What would realistically make life better in your household? I assume your parents aren't going to split up and they love you and don't want you gone. Will your bio parents make the rules and leave the step out of the discipline for your sake? It won't hurt to ask.
It's really hard to say without knowing your problem. Usually family isn't about what is legal, but what will help the family and what keeps it in order. Kids are supposed to listen to their parents. If he is telling you to do something bad, tell your mom.
In a way yes and no. If youre over age, and living on your own, then no. But if youre underage, living under his roof, yes. Even if youre overage he can, if you are living under his roof. Its common respect, whether he has adopted you or not.