Bad Girls Don't Die Series by Katie Alender

 When Alexis's little sister Kasey becomes obsessed with an antique doll, Alexis thinks nothing of it. Kasey is a weird kid. Period. Alexis is considered weird, too, by the kids in her high school, by her parents, even by her own Goth friends. Things get weirder, though, when the old house they live in starts changing. 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. Kasey is changing, too. Her blue eyes go green and she speaks in old-fashioned language, then forgets chunks of time.
Most disturbing of all is the dangerous new chip on Kasey's shoulder. The formerly gentle, doll-loving child is gone, and the new Kasey is angry. Alexis is the only one who can stop her sister — but what if that green-eyed girl isn't even Kasey anymore?

Alexis is the last girl you'd expect to sell her soul for beauty and popularity. After all, she already has everything she needs--an adorable boyfriend, the perfect best friend, and a family that finally seems to be healing after her sister Kasey's unfortunate possession by an evil spirit. But then Kasey tells Alexis about the mysterious new club she has joined, and the oath she has taken to someone named Aralt. Worried that Kasey's gotten in over her head again, Alexis and her best friend Megan decide to investigate by joining the Sunshine Club, too. At first, their connection with Aralt seems harmless. Alexis tries a new -- normal-- look, and finds herself reveling in her elegance and success.
Still, despite the Sunshine Club's outward perfection, the group is crumbling from within, and soon Alexis finds herself battling her own personal demons. She can hardly even 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?