Please Help Me I don't know what to do!?

I have been best friends with this girl for the lat 10 years. One of the girls in our group has hardly been out with us in the last year as she has a new set of friends. In the last week or so this girl has started hanging around with us again. My best friend knows I can't stand her. I have told her this but she just says..well don't come out then. She is deciding 2 hang around with her even though i'm always there for her and this other girl comes and goes as she pleases. I have been going out with them just to please my friend but I can't stand her! why is she choosing this girl over me and what am I supposed to do about it? I can't stop being friends with her because like I said she's my best friend and it would also make it awkward for the rest of the group...