Also: My naturally suspicious nature has me wondering if she didn’t see your promotion coming—the one that made you her immediate supervisor—and the flirting always was and still is an insurance policy that protects her from being fired. Right now, you’re not going to fire her because you want to fuck her, FIS, and once you’ve fucked her, you won’t be able to fire her because she’ll be able to accuse you of sexual harassment. See how that works?
My advice: Go to your boss and tell him that a little innocent co-equal-co-worker-to-co-equal-co-worker flirtation has been queered by your promotion, and it would be a sensitive situation even if the other employee was a good worker. But she’s a shit worker, and if she doesn’t shape up, someone is going to have to fire her.
And that someone can’t be you.
Ask your boss to place this girl under someone else, FIS, and after this girl’s new immediate supervisor fires her, you’ll quickly find out if she was ever really interested in being “placed” under you at all.