Boyfriend's best friend is a girl

My Boyfriend's best friend is a girl. They have a history when it comes to sexual activity, but they are just friends now. When we hang out with her, they can get a bit flirty, but I always thought it was just how they acted together because they're just so close. However, a numerous amount of people came up to me and asked me why he is always flirting with her. What should I do? Should I be worried?

How far does their sexual history go? Did they kiss? Have Sex? and How long ago was it?

If you are his girlfriend, you need to be his priority. This is my opinion and many may not share it, but If there is a past, especially a sexual one, there is no need for him to have her as such a "close" friend. I've been there and been on both sides of the coin...it has always been a situation that is unfair to the new party coming in as a new girlfriend or boyfriend. To me, your partner is the one you need to be close with, all others can be friends, but not best friends and most definitely, not flirty, that's disrespectful and I personally would bring it up or move on. I believe it's a manipulation tactic that leaves the new party feeling like they are on the outside looking in. Hope this helps.