Sisters of Misery — Megan Kelley Hall

There are some girls who have everything...
She has the right clothes, the right friends, and the right last name, but fifteen-year-old Maddie Crane sometimes feels like an outsider in her clique in the wealthy, seaside town of Hawthorne, Massachusetts. And when her gorgeous, eccentric cousin Cordelia LeClaire moves to town, Maddie is drawn toward her ethereal, magical spirit and teeters even more toward the edge of her friends' tightly-knit circle...

Then there are the jealous ones...
Kate Endicott and the Sisters of Misery--a secret clique of the most popular, powerful girls in school--are less than thrilled by Cordelia's arrival. When Kate's on-again, off-again boyfriend Trevor takes an interest in Cordelia, the Sisters of Misery become determined to make her pay...

Now Maddie must choose her loyalty to her cousin and the wrath of the Sisters of Misery...