it may be imoral, but carry on keeping tabs on her convos & talk to her friends who you can trust, about things.
if her friends do say something you don't like, then confront your sister, calmly, & just talk to her. if its drugs there must be a reason for wanting them, if so, why? if sex, then explain that she is too young & no ones worth it for her to make a mess of her life at such a young age...
i myself am 23 & have a 16 (nearly 17) yr old sister & every 2 week she has a new boyfriend & i'm always funny about it.
if it's boys dragging her into this kind of life, then go after them, & threaten the shit out of them, without hitting. trust me, this works. my sister got loads of crap from these two lads, i had enough of it, my dad had enough. so me, him & a mate of mine (steve) went after them in car & after nearly running them over, we had a few 'words' with them. since then, they have been ok...
but as ppl have said, if you are truly worried about things, then stopping her access to the internet could make things worse... has she got a mobile? and she cant stop in the house forever...
...all i can suggest at first is to have a quiet word with her. or if you have a female cousin who is close to you could maybe do it. depends how comfortable you both would feel.
My name is Sam Fisher
I used to be a hero - now I'm a wanted man.
I used to hunt terrorists - now I am one.
I used to take orders - now I execute them.
I've become, a Double Agent!