My "friend" is is stealing my best friend?

Me and my best friend are pretty close, we've always done everything together, but there's a girl, me and my best friend both don't like her, but my best friend has to pretend to like her because their parents are friends. This girl is ALWAYS trying to steal my best friend, me and my best friend always talk about going somewhere like the movies or the mall or something, and whenever she hears us, she just invites herself to go, its so annoying because when she does come with us, she always talks to her nonstop and won't let me say a word to her. This girl also makes plans with my best friend without telling me all the time, and I'm so sick of dealing with her. What do I do?