If you look closely at the first chapter, the ending of the book is not so random, and even almost implied Anna states that "See, unlike the rest of the free world, I didn't get here by accident. And if your parents have you for a reason, then that reason better exist. Because once it's gone, so are you." The fridge brilliance comes in when you get to the end of the book. Anna wins the case, and doesn't have to donate her kidney to Kate. But her entire reason for existing was to basically be a bag of spare parts for Kate. So when she won the case, her reason for existing was gone, and so was she. It seems like a simple, meaningless throwaway line at first, but is actually very important and telling.
The very beginning of the book is a flashback of a three-year-old girl trying to suffocate her sister and being stopped by her dad, and she concludes by saying "In the end, I didn't kill my sister. She did it all on her own. Or at least that is what I tell myself." The implication seems to be that Anna's the narrator here, but at the end, after Anna dies and Kate lives, it seems that Kate's actually the narrator of this passage.
In the book, after winning her lawsuit, Anna tells Campbell shortly before her death that she plans to give a kidney to Kate anyway. It seems like a "Shaggy Dog" Story type of ending for her to do what her parents wanted her to do after spending the whole story fighting for her right to refuse, but there's actually a kind of symmetry to it: Kate was willing to sacrifice her life for Anna's freedom, and once Anna has said freedom, she in turn effectively decides to return the sacrifice by giving Kate a chance to live (and of course the sacrifice is that much more meaningful because Anna is choosing it of her own accord). Of course, it turns into a Shoot the Shaggy Dog with Anna's death making the whole thing moot.
In Greek mythology, the princess Andromeda becomes a sacrifice to angry gods. In this novel, the character Andromeda (Anna) is born to be a sacrifice, albeit in little pieces, to her sister.
Fridge Logic
Why is someone with uncontrolled epilepsy allowed to drive?
People with epilepsy are allowed to drive if they're declared fit by a doctor and are seizure free for at least a year.
This is true, but Campbell's epilepsy is pretty clearly not under control given that he has a major seizure over the course of the book and, probably even more significantly, that he uses a seizure dog, which indicates this was not a random anomaly (it would be extremely abnormal for him to keep a service dog around just in case he has a seizure if the risk was that remote; the use of a service dog indicates a condition that needs to be actively managed by that method). The author probably just didn't think that part through.
Why is a child whose sole reason to exist is spare bits for her dying sister not taken away by CPS?
Because the family are clearly at least upper middle class (lawyer mother, fireman father) and not the kind CPS usually checks on - unless they're given a call. And the way it's framed is Anna willingly donating small things for Kate, which she seemed content to keep doing until the possibility of a kidney transplant was on the table. For example, donating bone marrow is a safe enough procedure that won't affect the donor. It's perfectly legal, provided there are no adults who can donate to the patient. The ethical issue is of course whether children can consent, as it has to be legally approved by the parents. Strictly speaking it's considered okay as long as the benefits to the patient outweigh the risks to the donor. The child's decisions are taken into account, and for the most part Anna is a willing donor. It's not until the kidney transplant is proposed that she now has second thoughts.
And for what it's worth, Anna wasn't created solely to be 'spare bits'. The only planned donation was an umbilical cord one, which the doctors hoped would work. It worked for a while but then Kate relapsed several years later, so the decision to use Anna to donate was made then. So the issue was very complicated.
On top of this, the family doesn't literally treat Anna like her sole purpose in life is to be a donor for Kate. That's not to say that what's being asked of her is reasonable, but it's not like they forget about her or pretend she doesn't exist except when they need her as a donor; Brian and Sara do love Anna and they're good parents to her for the most part, they just have one giant blind spot around Kate's illness (which seems like a case where their terror of Kate dying is impeding their ability to see any other concerns, rather than a sign that they consider Anna to exist solely for the purpose of donating to Kate). Even if someone did contact CPS, it would likely be a low-priority case, and when they did get around to it, there's not a whole lot they'd be able to do; most social workers would likely be reluctant to take Anna away from a home where she was generally well-cared for over this one thing, especially if Anna herself didn't want to go (and there's no indication Anna would want something so drastic), and most of their other tools (like mandatory parenting classes) wouldn't really fit this particular situation.
