She probly asked her not to tell cuz she thought you would flip. Which you kind of have
You're not gonna make her stop smoking by goin mad at her, that'll just make her feel bad and in the future she won't EVER come to you when she needs help. You got to accept that she's growing up and over the next 4/5 years will no doubt experiment with copious amounts of substances, and sex, just like every1 has done, but instead of critisizing(sp?) her just keep in mind shes the same person, smoking, drinking, having sex don't really change the person you are, but it will make ppl look at you differently, either with respect or remorse. I bet the last thing she wants is her big sister flying off the wall and treating her differently jus cause she likes to smoke the occassional fag. Just educate her on the facts of drugs and sex and let her make her own decision, cuz children and ppl her age aint as stupid as every1 makes out, they're just ignorant (not in the bad way) and need to learn. You learn through reflecting on mistakes, which the majority of the time involves you getting in to some kind of pain and I understand that as some1 who loves her u don't wanna see her get fucked up/hurt.
I would chill with her and see if she would like to read up about sum of the shit baccy does to u, and if you would in the future like to smoke weed with her, wash out that shite that her grandparents put in to her skull.
Hope u can figure this one out, seeing as its a delicate situation.