What is the significance of the title?

In this novel, Kate Fitzgerald has deteriorating health and her parents decide to have another sibling as a savior to aid in donating cells as well as organs, Anna is thus born. Anna is born tasked with the responsibility of saving her sister’s life by donating blood cells to treat her leukemia and donate organs such as a kidney. In this way, Anna becomes her sister’s savior and the title is thus an allusion to this fact.