Well.... the fact that he's 12 years old speaks volumes... He's obviously not mature enough to approach it as seriously as you probably are.
I'd hate to say it this way... but... that's kind of what you get for being in a band with a 12 year old... hahaha. Just kidding.
All bands have their little moments of butting heads. It's to be expected. With multiple people come multiple personalities and perspectives ands you're almost never gauranteed to have the same views on everything. The good bands, the ones that make it, are the ones that can work through that... Obviously, even for those kinds of bands it gets to a point either creatively or personally where there are enough differences to break up.
What I'm collectively saying is you're going to have to work with him on it. Since he's your brother... I'd say sit him down and try to have a serious convo with him about it. Big brothers can be pretty influencial, if you just sit down with him... outside the band practice area... and have a private face to face convo with him about it he may see the light and try to impress you by throwing out the ego. The worst thing you can do is approach him in front of the band about it because that puts him on the immediate defensive about the issue and he's going to block out everythign you're saying and try to use his "But I'm the best you've got" attitude.
Those are my suggestions...