I've taught programming, part time in an elementary setting, for years, and have been a real live stay at home dad for almost 20 years. I know code and kids. Kids learn people first, things later. So teach what *you* are passionate about. A kid will spot bullshit a mile away. You can trust that your little brother knows you better than you would like. Your little brother isn't going to be learning programming, he will be learning what you like. It's the "what you like" part that is most important.
Don't worry about what language to use, all languages suck about the same amount, just in different ways. C is fine, kids have a wonderful ability with language, any language. Any kid under 14 (puberty is the dividing line) or so will pick up the syntax of any language in a few weeks. Arbitrary and weird is fine, they just go with it. In my experience kids can learn either top down or bottom up, but they have a more or less fixed attention span. You have something like 20 - 40 minutes before he will start to get antsy. (YMMV) Regular times for set amounts of time work best. He knows that Monday at 6 big brother will give him a lesson and answer that question that has been bugging him for days, or years or centuries, they're all about the same. If in 6 months he would rather find something else to do, then consider dropping it. But remember!! The discipline in a 12 year olds life is external, not internal! That really doesn't start kicking in until HIgh School.
One thing that will kill this is forgetting that he is 12, below a certain but unknown age kids just can't get certain things. You just don't know what, exactly. Just because you got it at 12 doesn't mean he will. He may get different things. Then they just go to sleep and wake up and suddenly get it. They will often deny that they ever didn't get it, it is now natural and part of them. It's magic and frustrating as hell.