The novel establishes the theme of family from the opening scene, as Mr. Penderwick, lost without his map, navigates a car full of grumpy kids and one queasy dog, immediately conveying this endearingly familiar family dynamic. In her portrait of one three-week family vacation, Birdsall highlights the beauty of family through the Penderwick sisters and their father. Each member of the family has a distinct personality, yet the individuals remain close to each other and hold a strong loyalty to their family name.

Their mother’s absence looms over the narrative, but the story doesn’t focus on their grief. Instead, the author elevates the bonds among the sisters as they fill in the gaps left by the loss of a parent and work together to keep their family strong. An important part of their success as a family is cooperation and holding meetings to discuss how to manage sticky situations: “A MOOPS was a Meeting Of Older Penderwick Sisters. Rosalind, Skye, and Jane called it MOOPS to keep Mr. Penderwick from knowing what they were talking about” (30). Although the sisters often hide information from their father and sometimes argue with one another, they prioritize unity when it matters, like in caring for Batty and defending their family.