Brugman, Alyssa. Being Bindy. 2004.
This book was just released in the US with a different cover.
Bindy is in 8th grade and things start to change. Her best friend Janey was like a sister to her, always spending lots of time over at her house. Now, she has started hanging out with Hannah. Now Janey has started to dress and act differently. Eventually, she drops Janey altogether. Then, Bindy's father and Janey's mother start to date. Together, the girls agree they have to break them up because they don't want to end up being sisters, but it is really more Janey's idea and it sort of falls apart anyway. Meanwhile, Bindy is trying to navigate HS without friends and also her relationship with her distant mother.
I recommend this book for a middle school audience or perhaps 9th grade in HS. The characters are younger and act like it so older teens would't enjoy reading it.