Okies.
Right, at the end of the day, you aint gonna stop her from doing anything - sad, but it's a fact. Therefore, what you must do is be her teacher, rather than run the risk that other, less caring people, will.
So, why not take your sister out somewhere (I dunno where, but somewhere where you can have her undivided attention), and tell her the facts. Get as much information as you can from the net, and print it off (if you can) -and talk to her, like an adult, without trying to act as if you are forbidding her to do it - that will only alienate her from you - the last thing that youy need to do!
Make her feel at ease with the information that you are telling her - ie, explain things fully, and without glossing over anything or trying to frighten her with it.

At the end of the day, you NEED her to trust you and think of you as a person who is on her side - this is essential for the future. The thing that you are trying to acheive here, is TRUST from her, so that you can be sure that she will come to you for advice.
There are alot of bad folk out there that will prey on her impressionable mind - better that you get in there first, yes?
Then, she can make an informed decision based on the CORRECT INFO that you have given her, rather than the typical bullshit that someone less desirable could tell her, ie, how many kids have gotten hooked on heroin, because they were told that it is harmless??

It's up to you, dude - if it were me, I'd want to be the one informing her of her choices, and not some low-life drug-pusher!

Hope you find a way, dude - I am already preparing myself to inform my own kids on the facts of such things, and they're only 6 & 4, at the mo... I want to be their teacher, not a scummy twat from the outside world