Sisters of Blood and Spirit

Wren Noble is dead—she was born that way. Vibrant, unlike other dead things, she craves those rare moments when her twin sister allows her to step inside her body and experience the world of the living. 
Lark Noble is alive but often feels she belongs in the muted Shadow Lands—the realm of the dead. Known as the crazy girl who talks to her dead sister, she doesn't exactly fit in with the living, though a recent suicide attempt and time in a psych ward have proved to her she's not ready to join her sister in the afterlife. 
Now the guy who saved Lark's life needs her to repay the favor. He and his friends have been marked for death by the malevolent spirit of a vicious and long-dead serial killer, and the twins—who should know better than to mess with the dead—may be their only hope of staying alive.