Thirteenth Child — Patricia C. Wrede

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.
With wit and wonder, Patricia Wrede creates an alternate history of westward expansion that will delight fans of both J. K. Rowling and Laura Ingalls Wilder.