Bad Girls Don't Die Series by Katie Alender

Alexis thought she led a typically dysfunctional high school existence. Dysfunctional like her parents' marriage; her doll-crazy twelve-year-old sister, Kasey; and even her own anti-social, anti-cheerleader attitude. When a family fight results in some tearful sisterly bonding, Alexis realizes that her life is creeping from dysfunction into danger. Kasey is acting stranger than ever: her blue eyes go green sometimes; she uses old-fashioned language; and she even loses track of chunks of time, claiming to know nothing about her strange behaviour  Their old house is changing, too. Doors open and close by themselves; water boils on the unlit stove; and an unplugged air conditioner turns the house cold enough to see their breath in.

Alexis wants to think that it's all in her head, but soon, what she liked to think of as silly parlor tricks are becoming life-threatening--to her, her family, and to her budding relationship with the class president. Alexis knows she's the only person who can stop Kasey -- but what if that green-eyed girl isn't even Kasey anymore?

Alexis is the last girl you'd expect to sell her soul. She already has everything she needs--an adorable boyfriend, the perfect best friend, and a little sister who's finally recovering after being possessed by an evil spirit, then institutionalized. 

Alexis is thrilled when her sister joins a club; new friends are just what Kasey needs. It's strange, though, to see how fast the girls in The Sunshine Club go from dorky and antisocial to gorgeous and popular. Soon Alexis learns that the girls have pledged an oath to a seemingly benevolent spirit named Aralt. Worried that Kasey's in over her head again, Alexis and her best friend Megan decide to investigate by joining the club themselves. At first, their connection with Aralt seems harmless. Alexis trades in her pink hair and punky clothes for a mainstream look, and quickly finds herself reveling in her newfound elegance and success. 

Instead of fighting off the supernatural, Alexis can hardly remember why she joined in the first place. Surely it wasn't to destroy Aralt...why would she hurt someone who has given her so much, and asked for so little in return?

It's been three months since Alexis helplessly witnessed Lydia Small's violent death, and all she wants is for her life to return to normal. 
But normal people don't see decaying bodies haunting photographs. Normal people don't have to deal with regular intrusions from Lydia's angry ghost, sometimes escalating to terrifying attacks.

At first, it seems that Lydia wants revenge on Alexis alone. But a girl from school disappears one night, and Alexis spots one of Lydia's signature yellow roses lying on the girl's dresser the next day. Soon, it becomes clear that several of Alexis's friends are in danger, and that she's the only person who can save them. But as she tries to intervene, Alexis realizes that her enemy is a much more powerful ghost than she's ever faced before... and that its fate is tied to hers in ways she couldn't possibly imagine.

Not even in her worst nightmares.