1. She sounds too immature to be dating (although her evil, which comes with the onset of puberty, sounds like it's coming along nicely).
2. You are not her chauffeur. If she's planning to blackmail you, don't let her. "I'll tell <insert parent/guardian> you have <insert embarrassing/banned item>!" Get rid of it, or hand it off to a friend for safekeeping. If it's about something you've done, come clean, unless you ran over some dude after drinking and threw him in the ocean... Keep that one to yourself.
3. I have a younger sister, we fought like mad at the start of highschool, and when I got my license. It'll get better eventually. Set some limits with her, as in "this is what I will do, this is what I won't. Deal with it, ask someone else to drive you, or walk."
4. She's 14, girls still in the early parts of puberty (and often after) are evil. Pure evil. Don't let what she says get to you, she's probably covering her insecurities by poking at yours. It's probably better to ignore it.
5. If all else fails... Whack her on the head, lock her in a closet, have a nice quiet day to yourself.