ya book about two childhood best friends

This book was about two female best friends who met when they were very young at school (I think it was a private school). The main character (one of the best friends, I think her name started with a Z) has a crush on her best friend's older brother and promises herself that she'll marry him one day. The best friends begin to grow up and get into bad situations, I remember specifically they stole nail polish from a beauty store once. I think they begin to grow apart but are still friends. Eventually, her best friend's brother becomes a cop, and the night of a festival he shoots and kills someone with a brain injury, and she deals with the aftermath of it. Please help me find this book! It's a beautiful story.