Should I be jealous they are "bffs"? She's just such a whore and flirt whore!!!?

I also have a guy that's my absolute best friend. We really grew up together, our parets are bffs, and we have such a close bond, but it's not like we talk all the time. But my guy that I may start dating is "bffs" with this girl who he just started talking to this year. She's attractive according to many guys. And she's a flirt whore and just a whore!! He insists they are best friends, but I hate her. Idk if she hates me, but I think she may flirt with him just to get him away from me. Should I be this jealous? Am I being a hipocrite? I trust him, just not her!!