May not necessarily be considered moral imperative, but in America the government has an obligation to explain why they are preventing a freedom. I can understand such drugs as meth, but marijuana is really not that dangerous of a drug.
Anyway, on topic. OP it isn't really cool to look through your sister's phone. Don't confront her aggressively, but if you can get some information about how long she's been doing it and such, talk to her. Do NOT go to your parents. All you will do is drive a rift between you and her. As long as she is doing it with a fine crowd, she'll be fine.