In Jodi Picoult's "My Sister's Keeper," the theme of promises and their repercussions is central to the narrative. The phrase "Only cheats go back on a promise" encapsulates the idea that breaking promises, especially to loved ones, can have deep ethical and emotional ramifications. This reflects the characters' struggles as they try to navigate complicated family dynamics and make difficult decisions regarding loyalty, life, and morality.

The quote serves as a moral compass for the characters, highlighting the importance of integrity and commitment. As the story unfolds, the conflicts arising from broken promises challenge the characters to confront their values and the impact of their choices. Ultimately, it examines the complexities of familial obligations and the pain that can result from betrayal.