Once A Witch Witch Series, Book 1.

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, ul...