I remember that film very well, and remember the first time I saw it on a local PBS station. That had to be twenty years ago, and the film had several years on it even then. It was an "after school" special. Yeah, I thought it was pretty special myself.It was quite amazing. The premise, as Jason noted, is; nerdy girl visits cool cousin, discovers that cool cousin smokes (and does so wonderfully), then wrestles with the decision to give smoking a try herself.
Seems she does indeed try smoking but turns against it quickly. The predictable plot/moral of the story is completely blown away by the gorgeous blonde cousin who hid cigarettes (I believe B&H 100s) in a large flower vase in her bedroom. This girl put on one heck of a show, and again as Jason stated, she loved performing with a cigarette in front of the mirror. Pushed for a guess, I'd say smoke-chick had at least three smoking scenes, all of them incredible.
I have no idea who the actress was or how old she might have been, but this was no poser, she was undeniably IRL. She was supposed to be 13/14, but I imagine she was 18 minimum. Then again, maybe not. Remember, this thing was shot in the 80s, it was a whole different environment back then.
One thing I was convinced of was this: Whoever produced and or conceived this piece had the SF and was looking for a way to legitimize young smokers on screen. Whoever they are, they succeeded.
FWIW, I did find a respectable clip from this thing on YT a year or two ago. It may still be there, but you'll have to work to find it.