Jessica has a little sister, Emma. The two have a very typical sister relationship: close, friendly, antagonistic.
The book is really a series of brief poems that build into a larger story of the girls' relationship: the annoyance of a pesky little sister going through your stuff, the delight in sharing an old favorite book together, the challenge of splitting a piece of pie so it's really fair. But later in the book it's darker as Emma breaks her arm and Jessica worries she might have prevented the accident.
While the book is subtitled "Big Sister Poems," really they're simply sister poems. Whether you're the little or the big sister, you'll find something familiar here. I think kids will enjoy finding bits of themselves in both Emma and Jessica and revisiting the ups and downs of sibling love and rivalry.