What is the exact model number of your router? I know you don't know it. Go pick it up and check.
All you need here is some decent QoS rules. Basically, you tell the router to put all of your internet traffic as high priority and/or all of his traffic as low priority. Then it doesn't matter what he does, the router makes sure your traffic goes through, even if that cuts into your brother's traffic. (which is what you want.)
I ask about the model number, because I want to know if your router supports alternative firmwares, like Tomato or DD-WRT. I know that Tomato has the QoS feature and I'm pretty sure DD-WRT has it too. And you can put a password on the router too just in case he figures this out.