1) I'm 19. I picked up smoking with one friend that I saw once or twice a week. (I only smoked so I could blow smoke rings!)
Did that for a month or two, got bored.
If you don't want to smoke, you won't.
It's a CHOICE thing. It doesn't matter how many scary pictures of brains getting cut open we see (At least that's what the G'vnt does here) - people who want to smoke will.
They're quite informed about the risks.
Summary - let your brother do it. Say you disapprove or whatever, but don't turn it into a situation where he starts sneaking off to smoke them
