I had a little trouble following your post (sorry!). Am I right in understanding that there are two kinds of lying going on?
1. She's lying to cover to avoid getting in trouble. She said something rude about a classmate, got called on it, and is denying it to stay out of trouble.
2. She's making false allegations about her teachers, her sisters, etc.
Both are honestly common with teenage girls, but # 2 is more troubling than # 1.
If she's just lying to avoid getting in trouble, double the punishment. Say, "You did two things that tell me I can't really trust you. First, you said X about your friend, and second you denied it. If you just had said X, I would have taken away your Internet privileges [or whatever] for two weeks, but because you lied to me, I need to take them away for a month." And then, look out for instances of honesty, even small ones, and praise the hell out of them: "Hey, did you just admit to eating the last of the cereal? Honesty! That's what I like to see!"
However, if she's also making false allegations, I would get her into counseling. Again, this is common, but it's a sign of more serious underlying issues. A good adolescent psychologist could probably do a world of good.
Best wishes,
Mira