Content Warning: This section of the guide includes discussions of child abuse, sexual abuse, grooming, and suicidal ideation.

The title reveals the key theme, with Summer Sisters indicating the fluidity of families. The first summer spent together at the Vineyard fastens Caitlin and Vix for life and changes their friendship to a form of sisterhood. As the summer is a season, it doesn’t last forever—it comes and goes, and the impermanence of families produces vulnerability. The narrator explains, “After the first summer, Vix hadn’t expected Caitlin to invite her back to the Vineyard, and when she did, Vix worried that her mother wouldn’t let her go” (61). Vix’s situation is unstable and beyond her control—Caitlin and Tawny determine who will constitute her family. Caitlin advances the fluidity of families when she tells Vix, “Even though we’re summer sisters and always will be, I have another life at Mountain Day, a life apart from the two of us” (113). Caitlin and Vix remain “summer sisters,” yet Caitlin, beyond the Vineyard, turns into someone else. She’s Vix’s sister, but she’s also not her sister: She’s in flux.

Vix propels the theme when the Chappaquiddick birthday party leads to a fight that compels Vix to live on Trisha’s boat.