I would argue that the story plot of Patricia Reilly Giff's Pictures Of Hollis Woods suggests themes of family and loyalty.
Having grown up in the foster care system, Hollis has no real understanding of family until she is fostered by the Regan family, with whom Hollis immediately feels safe. Over the course of a summer, she becomes close to the Regan family, who soon ask if they can adopt her. Her sense of belonging, however, is still fragile enough that when her foster brother, Steven, is injured in a car crash that Hollis perceives to be her fault, she runs away. However, family ties run deep, even if they are not biological, and Hollis learns later on in the novel that Steven has been watching over her from a distance as she takes care of Josie at the Regan's summer house.
The theme of loyalty is evidenced by Hollis's growing concern for Josie. After being placed in Josie's care following her sudden departure from the Regans, Hollis realizes that Josie is slipping into dementia. As she is unable to bear the thought of leaving Josie after the social services agency learns that, thanks to her dementia, she hasn't always been ensuring that Hollis is in school, she makes a plan and runs away with Josie to the Regan's summer house. It is her sense of loyalty to someone who has been kind to her that renders Hollis unable to abandon Josie.