How old is she, to begin with? If she's young enough, she might just be playfully exploring in the only way she knows how to, without really looking to the consequences.
If she's old enough to know what she's doing, however, I can see where the discomfort is coming from. Next time she does it, ask her: "Why are you doing that? or "What are you trying to find out?" and confront her with her actions. If you aren't blunt then she might not realize that it's not a game, and it really is affecting you.
On the flip side, if she actually is doing it to hurt you, and it isn't just a game to you, I'd go to your parents. Younger siblings often don't listen to their older siblings just because, so you probably won't be able to do anything if she really does have malicious intent. Go to the people she can't ignore.

Good luck!