If they are just going to smoke weed, well you shouldn't be so worried about that but if you wana do something tell ur parents about that. But sharing this in mmoc is not the best way to help her if you must, imo.
I love these threads. I really don't care if someone wants to smoke pot, but encouraging people to do so and acting like it's a good thing is just beyond stupid. Do teenagers experiment with drugs? Yes. Should they? No. The only answer to that question is no, any other answer is ridiculous. At no point should anyone be encouraged to experiment with illegal drugs. Or legal drugs for that matter except under the guidance of a medical professional to find the right amounts to fix whatever ailment they're suffering from. I'd say the same thing if someone was encouraging someone to consume alcohol.
OP: The best thing you can do in your situation is to mention it to your parents. They're in a better situation than you to deal with it. Your sister will probably be pissed off at you about it, but it's likely for the best.
Go nuts. Completely harmless stuff.
"Can your blood atone for genocide, orc? Your Horde killed countless innocents with its rampage across Stormwind and Lordaeron. Do you really think you can just sweep all that away and cast aside your guilt so easily? No, your kind will never change, and I will never stop fighting you." - Grand Admiral Proudmoore
Addiction is an actual and real thing. Denying it is pretty much attempting to deny reality. The thing about marijuana is it does not have chemical addiction, but it does have behavioral addiction. Ie, the same kind of addiction that you get for things like gaming.