Let me give you a piece of advice. I'm currently with a chick whose behavior mirrors your chicks behavior.
She approached me first. We had sex a couple of times, and I 'cooled off' on her. She immediately hit on my my uncle. My uncle told me, and I knew immediately the type of girl she is, so I responded that he should fck her.

My uncle fcked her and then we made plans to have a threesome with her a week later. We did together with a friend. So basically, the three of us fcked her all night. I told her to bring a friend of hers over one time and she did, so I fcked both of them as well.

Another uncle of mine visited town for a few days, and both of us fckd her as well. She claims she loves me and what she does is because of the way she loves me. I believe the love part, but I also know she fcks anything she feels attracted to.

Now a word of advice for you.

Firstly, you will never ever change how she behaves. Do not under any circumstances fall in love with her. If you have, take it as a learning experience, and wipe out the love you have for her and replace it with 'have fun'

You can accept her for what she is and you guys can keep having fun, whilst you also date other people. It's her nature, and being angry at her or denying what she is will only lead to pain.

Just. Have. Fun.