Imagine for a moment that you were brainwashed, you wouldn't believe it, none of us would, but how can we check? Logic and reason.... the scientific method..critical thinking is how you go about checking. Your brother understands that you have been brainwashed. If he has used logic as you say, he would have considered the possibility of a god with out jumping to conclusions.
The best I can say in short is to try and look for reasons why your belief could be unwarranted, not just for reasons that you think justify the belief like most religious people do.
"The God delusion" by Richard Dawkins helped by partner break free. I urge you to read it.

