River Between Us by Richard Peck

The year is 1861. Civil War imminent and Tilly Pruitt's brother, Noah, is eager to go and fight on the side of the North. With her father long gone, Tilly, her sister, and their mother struggle to make ends meet and hold the dwindling Pruitt family together. Then one night a mysterious girl arrives on a steamboat bound for St. Louis. Delphine is unlike anyone the small river town has even seen. Mrs. Pruitt agrees to take Delphine and her dark, silent traveling companion in as boarders. No one in town knows what to make of the two strangers, and so the rumors fly. Is Delphine's companion a slave? Could they be spies for the South? Are the Pruitts traitors? A masterful tale of mystery and war, and a breathtaking portrait of the lifelong impact one person can have on another. - from Amzon