Balefire

Balefire by Cate Tiernan

Clio Martin and Thais Allard are twins, separated at birth by their grandmother, Petra, to protect them from the questionable machinations of people within her own coven. Thais goes with her father to live in Connecticut, while Clio remains with Petra in New Orleans. But when they are 17, their father is killed in a mysterious accident, and Thais finds herself in the hands of strangers in a city she never thought she had any connection to. Running into their twin the first day of school is the least of the surprises the girls face as they both discover their family's true history, one that goes back further than either of them ever imagined.

Thais soon learns that she and the twin she never knew come from a family of witches, that she possesses astonishing powers, and that she, along with Clio, has a key role in Balefire, the coven she was born into.
Fiery Clio is less than thrilled to have to share the spotlight, but the twins must learn to combine their powers in order to complete a rite that will transform their lives and the coven forever