Like Lou, Will has sister issues. His little sister Georgina appears halfway through the novel and leaves a trail of inter-family chaos in her wake.
In this instance, however, the roles are reversed—Georgina feels like Will is the family favorite, rather than the other way around. This feeling is amplified after Will's accident, because his parents have to start paying more attention to him than ever. A little gross on her part, right? As all of us with siblings know, however, this stuff can get pretty complicated at times, and childhood issues tend to flare up when we least expect them. Judge not lest ye be judged, and all that.
Georgina is also passionately opposed to Will's plan to commit assisted suicide. She's furious that her parents would be party to such a heinous act. But here's the thing—she doesn't live with Will. She doesn't see what he goes through every day. How can she pretend to know better than they do?
For example, check out how Georgina rejects Mrs. Traynor's suggestion that she accompany Lou and Will on their trip: "'It's a really good job, Mummy. It's the one I've been working toward for the last two years.' She glanced over at her father. 'I can't put my whole life on hold just because of Will's mental state'" (10.35).
Well, you heard it from the horse's mouth: Georgina doesn't want to put her life on hold to deal with Will—something that her parents have been doing for two years. They don't want to see Will end his life any more than she does, but they know firsthand the complicated reasons that make it almost an inevitability.