..and I'm pretty pissed. 
 
Now, it's not marijuana or anything, just regular cigarettes (so it's actually worse) . Before she never really smoked, only very rarely at a few parties here and there, but now she has started to buy her own packs (not cartons, thank god, but a pack is still a pack too many).  
 
The thing is, I'm one of those "protective" brothers, meaning that if I sense my sister is in any kind of trouble, I prevent the trouble from happening. Even though she is 5 years older than me, I still feel as the older sibling, which is pretty nerve racking to say the least. Unfortunately, she is the kind of person that will indulge in things in order to "fit in", and the excuse she always gives me is "well, everyone else was doing it."  
 
The good thing is is that she's already become uncomfortable smoking around me when with her friends, but I know that once I'm out of the room she doesn't care as much, if at all. I've already discussed this new habit with her, but she merely makes excuses and tells me to stop trying to be like our dad (our parents don't know a thing by the way).  
 
Here's the difficult part: I'm living with her now, and I don't want things to turn stale. I've thought of simply forbidding her to smoke, but I honestly don't think that will do any good. I've smoked for a year, maybe a year and a half, and see that as the greatest regret in my life thus far, so this affects me quite closely.
 
I don't want to make this into a big thing or anything, I'm not asking for life lessons here, but since I can't turn to my parents on this one (long story), I'm just curious what some of you would do if you were in my situation. 
  
EDIT: sorry for any confusion, but I am no longer a smoker. Haven't had a cigarette for 8 months now.
  
Thank you,
 -Jolly_Lolly