My Suposedly "Best Friend" isn't exactly acting like one.?

Me and my "best friend" have been friends since kindergarten but recently she's been hanging out with my sister a lot. And not just a bit but like every day. They always go to the mall and stuff and then sometimes as a lat minute thought she asks if i want to come and now i usually don't go. As I am writing this they are at a party. Originally we were supposed to go to mosaic tonight together but they decided to go to a party instead. They also won't let me go because they are going to get drunk and my friend won't let me even though I'm the same age as her. I just want to know how to stop being friends with her without actually saying anything. I've asked this question before and people said to get her out of my daily life but next year we're going to the same high-school so that won't really work.