I dont have a younger sister but let me think what i would do in this situation....

You can't let her know you've been snooping, she will run away from home

I dont know how well you and yur sis get along but perhaps spend more time with her, and talk to her about boys and stuff, she what she says.  If she tells u that shes talking to people on the internet try and find out more, let her know it's not safe. 

Dont whatever u do argue with her, because i do have an older sister and know you can't win. 

If she lies to you and doesn't tell you anything then sneak around it,  say u accidentally saw her msn and were a bit worried or something. 

Or just tell her that you know what she gets up to on the internet and if it continues your going to make her life a living hell.  Not like she can tell on you if ur parents feel the same way?  might even make them do some parenting..

btw this is so much simpler if its a brother isn't it?

if your dad caught your brother trying to screw somebody he met online he'd just wait for him to come home and beat the snot out of him

http://www.youtube.com/watch?v=FH6JaNMAmuU

@gamer

how would they explain them knowing all that stuff?

Definitely don't threaten or try to pen her up. Rather, talk to her and such, gain her trust and give her yours. Teenagers can be volatile and unpredictable and make many mistakes, but more often than not, they learn if taught properly. Just don't, as Dimpel said, push her away; you want to be able to have her trust you enough to take your word seriously. She's unlikely to trust you if you let her know you've been spying on her and that you want to threaten her with it. I have two younger sisters and I've had to deal with such things before, good luck!