Once A Witch, by Carolyn MacCullough

Tamsin Greene has grown up in a family of witches, magic is commonplace in her life. Need a love potion? Just ask her mother. Is someone making your life miserable? One of her crazy extended family members can just send a well-placed hex their way.
But Tamsin? She can't do a thing, she's an outsider. Talentless. The one who was supposed to be the most powerful in the family never manifested, she doesn't have a drop of magic to call her own.
So, when a mysterious stranger mistakes her for her talented older sister, asking for her help to track down an old family heirloom Tasmin can't help but play along and enjoy feeling special for once. But this stranger isn't who she thinks he is, and the heirloom she helps him find is one that will unlock her powers and cause destruction for everyone she loves.