Bufflehead Sisters — Patricia J. DeLois

As an only child, Janet longs for a sister her parents are unable to give her. In kindergarten she meets Sophie, a strange and imaginative girl with a troubled family life. As friendship grows between the two girls, Janet believes her prayers have been answered, especially when members of her family embrace Sophie as one of their own.

Sophie's troubles continue to follow her through high school, and Janet stands by her "sister" until, in adulthood, she learns of a devastating secret Sophie has kept from her. Janet's world is turned upside down-and she discovers there may be a limit to what sisters should share.