Thirteenth Child (Frontier Magic, #1)

Eff was born a thirteenth child. Her twin brother, Lan, is the seventh son of a seventh son. This means he's supposed to possess amazing talent -- and she's supposed to bring only bad things to her family and her town. Undeterred, her family moves to the frontier, where her father will be a professor of magic at a school perilously close to the magical divide that separates settlers from the beasts of the wild.