The main theme in Jacob Have I Loved would be sibling rivalry and jealousy. In fact, the title of the book originates from the Biblical sibling rivalry between Esau and Jacob.

In the story, Sara Louise has always felt overshadowed by her more beautiful and accomplished sister, Caroline. While Caroline excels in just about everything in life (she is even socially popular), Louise has always felt like an awkward shadow. She befriends a local boy, named McCall Purnell; they both go crabbing together. Call doesn't seem to mind that Sara Louise is a tomboy. Both sisters have an uneasy relationship as a result of Sara Louise' self-consciousness and self-esteem issues. Caroline's patronizing attitude toward Sara Louise also renders her an unsympathetic character in her sister's eyes. Meanwhile, both girls grow up, and Caroline eventually wins a scholarship to study music at the Julliard School Of Music.

At this point, Sara Louise is left wondering whether she will ever make anything of herself. When Call announces his engagement to the beauteous and talented Caroline, Sara Louise finds herself surprisingly pained by the announcement; in a moment of courage, she decides that she will go to college after all. After college, she works as a nurse and midwife in a small town. Sara Louise eventually marries a widowed father. She eventually discovers that she does not have to live in her sister's shadow and that she possesses the power to decide her own destiny and her own happiness in life.