"The Lion, the Witch and the Wardrobe" by C.S. Lewis is the tale of four siblings who get transported to the magical land of Narnia. Although the story is a fantasy, it has a strong moral compass and is concerned with the triumph of good over evil. Two of the main characters, Susan and Lucy, are sisters, which will make this book appealing for sisters to read together.
The "Ramona" books are a series of stories by Beverly Cleary about a young girl named Ramona and her family. Ramona has an older sister named Beatrice, who is nicknamed Beezus. In "Beezus and Ramona," the girls don't always get along but manage to make up in the end. It accurately portrays the dynamics that can exist between an older and younger sister with humor, making it easy for real-life sisters to identify with the characters. In the book "Ramona Forever," Ramona gains a new baby sister.
"Little House on the Prairie" is the third book in the "Little House" series by Laura Ingalls Wilder. It is the semi-autobiographical story of Laura, who grew up in the late nineteenth century in frontier territory. In this book, the Ingalls--Ma and Pa, with Laura and her sister Mary and baby Carrie--have moved to Kansas, then known as the "Indian Territory." The family has a difficult time, having to go through illness and danger, while on the prairie. The book shows how the family needed to work together to forge a life.
"Little Women" is Louisa May Alcott's classic tale of four sisters in Massachusetts whose father is away from home serving as a chaplain during the Civil War. Each of the sisters is a vivid character in her own right and girls can alternately identify with Meg, Jo, Beth and Amy. While the sisters have their disagreements, they are usually the best of friends. The book focuses on the girls' struggles to grow up to be good, responsible women.