The Sisterhood of the Travelling Pants by Ann Bashares

Tibby, Bridget, Lena, and Carmen are best friends. The closest knit group known to man. They are sisters in every way but blood. They are each other's family when their families don't feel right.

But this summer, they're spreading out.

Lena is going to beautiful Greece
Bridget is going to hot Mexico
Carmen is going to South Carolina
Tibby, is well, she's staying home

It's only after they find a pair of pants that fits all of them. Bridget; tall and muscular, Lena; short and small, Tibby; skinny and lanky, and Carmen with her Puerto Rican butt. That they find a way to keep themselves together.

But is it enough?