[Lionheart]

Actually I'm with your parents here. If I were them I would do exactly the same thing (though I would probably straight out tell you no instead of letting you wait) You're 13 and M rating means you gotta at least be 17. Sure, ratings can be inaccurate but still, that's a 4 year difference and whoever rated that game had a reason to rate it as M. It may not be very apparent to you as you haven't been able to play that game you are talking about yet.
Not saying you need to wait till you are 17 to play M rated games, but it depends on how mature your parents think you are. If they think you can handle it they will probably allow it until then all you can do is wait.
Getting the game behind their backs would probably convince them of the oppisit so you definitely should not do that!
Also yes, maybe it has something to do with your brother. I wouldn't want someone to play games like that around a 4 year old. That's simply no good.