I DO keep my brother away from my rat, but my brother also gets home an hour and a half before I do, where he is apparently free to go into my room even though he is not allowed GRRR LITTLE BROTHERS ARE PAINS!!!
Get a keyed lock for your door. Or some sort of lock for the cage. Your brother is probably the one making the rat mean..probably teasing the poor thing.

I hope you take him to be neutered, and it helps. And even if it doesnt help, please keep him, and just keep your brother away.