Once a witch

"Born into a family of witches, seventeen-year-old Tamsin is raised believing that she alone lacks a magical "Talent," but when her beautiful and powerful sister is taken by an age-old rival of the family in an attempt to change the balance of power, Tamsin discovers her true destiny."@en
"Born into a family of witches, seventeen-year-old Tamsin is raised believing that she alone lacks a magical "Talent," but when her beautiful and powerful sister is taken by an age-old rival of the family in an attempt to change the balance of power, Tamsin discovers her true destiny."
"Tamsin Greene comes from a long line of witches, and she was supposed to be one of the most Talented among them. But Tamsin's magic never showed up. Now seventeen, Tamsin attends boarding school in Manhattan, far from her family. But when a handsome young professor mistakes her for her very Talented sister, Tamsin agrees to find a lost family heirloom for him. The search-and the stranger-will prove to be more sinister than they first appeared, ultimately sending Tamsin on a treasure hunt through time that will unlock the secret of her true identity, unearth the sins of her family, and unleash a power so vengeful that it could destroy them all. This is a spellbinding display of storytelling that will exhilarate, enthrall, and thoroughly enchant."@en