Brother and Weed.

Okay, so my brother used to smoke weed, but he had stopped right before school began. To start he is 17, and still smokes squares(cigerretes). But today he asked my cousin if she could lend him five dollars so he can buy some weed. I saw it and got pissed. I recently told my boyfriend about this, and i was saying that I couldn't tell my parents because my father has real bad medical problems, and my mom wouldn't be able to do anything anyway, so as I was telling him this, he told me that I should still tell my parents, but he doesn't understand that my parents CAN'T do anything. All my mom can do is yell at him, and we can never tell my dad, he doesn't even know my brother smokes squares.

So what I'm pretty much saying is...

1.Should I tell my parents?
2.Should I, ONCE AGAIN, confront my brother and tell him to stop?
3.Just leave it alone and hope he stops on his own??

Please Help.