Weed is a soft drug, it's not a hard drug with serious side effects like say... cocaine or amp. She will try various things out, and because of the social stigmata with drugs not *approved by the government* your doubting her character.
If she becomes addicted (I've never heard of people addicted purely to the drug itself, but the phys associated with it) and steals ect THEN you intervene...
I just think your worried about the legal aspects and attempting to be a brother, shes growing up and will rebuff any efforts to control her life. You can advise her however, but going to her parents will make her hate you and might even make the problem much worse.