Witch - Carolyn MacCullough

Vol I: Once a Witch
 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.

Vol II: Always a witch
The adventures of Tam and Gabriel continue with more time travel, Talents, spy work, and of course, the evil Knights.

Since the gripping conclusion of Once A Witch, Tamsin Greene has been haunted by her grandmother's prophecy that she will soon be forced to make a crucial decision—one so terrible that it could harm her family forever. When she discovers that her enemy, Alistair Knight, went back in time to Victorian-era New York in order to destroy her family, Tamsin is forced to follow him into the past. Stranded all alone in the nineteenth century, Tamsin soon finds herself disguised as a lady's maid in the terrifying mansion of the evil Knight family, avoiding the watchful eye of the vicious matron, La Spider, and fending off the advances of Liam Knight. As time runs out, both families square off in a thrilling display of magic. And to her horror, Tamsin finally understands the nature of her fateful choice.