by Peterne Van Arsdale
Set in a puritan-like land, Alys's family and all the village adults were taken by the soul eaters -- once human twins that now are life-craving demons. The surviving village kids are used by another village as shields against the evil.
Alys, once spared by the soul eaters, feels an uncanny connection to them. Terrified that she might be like them, Alys must face her darkest fears to stay human and resist the soul eaters' lure.
(Reminds me of a darker, YA version of The Girl Who Drank the Moon.)