Jesse's motivations. He was almost completely ignored after Kate was diagnosed and Anna was born because he wasn't a match for Kate and ended up destroying things because he'd never been able to fix them.
Anna's entire situation; being dehumanized by your own mother because she expects you to sacrifice your entire well-being to save your sister. It's worse in some ways for Jesse; since he's not a compatible donor, he's often pushed aside and without any attention completely.
Good god, the ending. It's especially heartbreaking because just before the accident, Anna had indicated that she intended to give Kate a kidney (the difference being that now it's her decision to make, not her parents'). Kate ends up surviving and going into remission from the transplant, which suggests that if not for the accident, Anna and Kate could both have lived.
The film
Sara breaking down in front of Kate, finally being forced to acknowledge Kate's choice to peacefully die instead of continuing to suffer, curling up into the hospital bed as Kate holds her. Then Anna's narration telling us that Kate died that night.
Kate choosing to die from her cancer than witness her family continue to mentally torture her younger sister by forcing her to sacrifice her well-being (and essentially her future) to let her continue living a painful existence.
