Know what this YA book is called ?

So there is this book I remember reading when I was 11/12 but I can't remember the title or author .

It was a Young Adult book it had two girls on the cover looking at pictures , one blonde the other brunette . The book was about them being long time friends who grow apart over time .

They first meet in school in kindergarten ( I think ) and one bites the other on the cheek . Yet they still become friends and at first the brunette is the popular one but over the years the blonde one becomes more popular .

Other details I remember the brunette has brothers and her grandma lives with her family and the blonde has money .

I don't remember the names :/

Also I am 25 so I think this book is from 1980-1999 .

This book has been in my mind and any help would be appreciated .