well by the time you're 15, chances are you've drank or done some kind of drug at least once. you really can't stop kids from doing things like this, they have to learn and think on their own. the most you can do is warn them about it.
Hey, if he wants do do that shit, fine, but I think his sister has a right to worry... I mean, that shit really fucks some people over, especially if you're not careful...