Thirteenth Child by Patricia C. Wrede

What if our history had magic in it? What would be different? Does a belief really mean that an innocent child can be bad or great because of the number of their birth order? Eff is a twin. She is the 13th child, but her twin brother Lan is the 7th son of a 7th son. She will be a bad seed and her brother will be a great magician, at least according to her uncle. To escape her uncles', aunts', and cousins' beliefs, her parents agree to let her Dad take a job as professor of Magic in a land grant college at the edge of the wilderness on the Mammoth River. It is a frontier town just like the ones in our history, but magic keeps the mythical beasts like dragons, unicorns,and woolly mammoths from the settlements. Adventures are always around the corner and danger is nearer than anyone wants to think. As Eff and Lan grow up, they learn that there is more than one type of magic. But which is best, the one that gave their relatives their beliefs and their father teaches, or the one a favorite teacher learned from her parents? With many twists, characters, and weird facts that seem so similar to our history, the tension builds until the their world starts to blow apart and the matter of who is the real powerful one is questioned in a fight to not die or see their father's death before their eyes. See if you can figure out what part of our history has been changed by just adding a little magic.