What to do in this situation?

Well man, it seems like a tough situation. The best thing I can tell you is to talk to your step-mom about what you want to do, if she says no, don't argue just deal with it for awhile, and do things around the house to show her you deserve this privileged. And when your brother does break the rules you have placed on the 360, just talk to him one on one respectfully, don't act angry just be chill, and if he blows you off talk to your step-mom about it.