This does not sound like an easy task, but I agree with birchlake, you must hide your medications, even if its under your mattress. You should confront your brother letting him get away with it is not the answer. Yes he will become defensive, but ignoring makes him think he can get away with it and he maybe trying it with others.
If you don't confront him, eventually he will get arrested from stealing or breaking in some place to get his fix. He may not be ready for help but he definitely needs some one to confront him.