All of her 13 year life, Anna has had to be there to take care of her sister. In fact, this is the reason she exists: when Kate gets sick, Anna donates bone marrow, blood, and other body tissues to fight off the effects of Kate's leukemia. But as their mother asks Anna to donate a kidney, something breaks, and Anna turns to a lawyer to get legal permission to say "no."
Picoult is an adult lit writer with a great grasp of how adults interract, but really not much of a sense of what makes teens tick. Anna and Kate swing between being portrayed as petulant brats and being simply young (but very well-spoken) adults. They talk like (very intellectual) grown ups and have very adult motivations for their behavior. Picoult has a great story here (albeit with a contrived ending) but no real sense of her younger characters. Read this as an adult book, not YA. And I'll wait for a good YA version of the story.