Anna’s wish to put her own interests first—specifically to live independently of Kate and to stop serving involuntarily as Kate’s donor—and her incompatible desire to put Kate’s interests first form the central conflict of the novel. The trial, which takes up a considerable portion of the novel’s plot, centers on resolving this conflict. For most of the trial’s length no easy distinction can be made between which is right and which is wrong. Anna has no legal obligation to donate her kidney, which would require surgery and carries a risk of health problems. Yet without Anna’s kidney, which Anna can live without, Kate will die. Several of the characters struggle throughout the book to determine which is the right solution, with different characters arguing different sides of the point, but no one can come up with an argument that settles the issue completely. Only when Anna reveals Kate’s wish to die, making it clear that even Kate does not want Anna’s kidney, does Judge DeSalvo issue a ruling.
Relationships between sisters appear repeatedly in the book, in the characters of Anna and Kate, Sara and Zanne, and Julia and Izzy. In each case, the sisters share an intense bond. Often they rely on each other for support, evident when Zanne visits to take care of Anna and Jesse, or in Julia allowing Izzy to move in with her. Sometimes that support just means one sister listening to the other’s thoughts or problems, as we see each pair of sisters do multiple times in the novel. In the case of Anna and Kate, however, their bond is both more intense and more strained than those of the other pairs of sisters because of Anna’s role as a donor to Kate. From birth, Anna has served as Kate’s savior in addition to being her sister. Both girls recognize this fact. Kate, though she often behaves as any big sister would, also shows immense gratitude to Anna. But Anna’s blood literally flows through Kate’s veins. Anna, for better and worse, feels so strongly connected to Kate that she compares their relationship to that of Siamese twins at one point. Her metaphor suggests that not just their lives, but also in a very real way their bodies as well, are interconnected.
Several characters in the story keep secrets from the rest of the world. Campbell hides his epilepsy and his real reasons for breaking off his relationship with Julia, Anna disguises her real motivation for filing her lawsuit, and Jesse conceals his acts of arson. In doing so, each character satisfies a deeper motive: Campbell doesn’t want to feel vulnerable or invite pity; Anna doesn’t want to reveal Kate’s wish to die; and Jesse doesn’t want to be found out, as much because he doesn’t want to have to discuss the feelings spurring his delinquency with his parents as because he doesn’t want to be punished. In each instance, the character prevents other people from knowing the real motivation for her or his actions, creating a discrepancy between what people think the motivation is and what the motivation actually is. Brian hints at this discrepancy when he speaks about dark matter, which, because it emits no light, cannot be seen directly, though it can be measured by the gravitational pull it has on the objects around it. In other words, the characters in the novel create a contrast between their appearances and their true feelings and actions.