Catherine's younger sister Sarah, or Sally, is a failure as a sibling, as least in terms of romantic literary sensibility as outlined by Jane Austen. Rather than being best friends, Sally and Catherine appear to have a fairly normal sister relationship. Sally doesn't care that Catherine is leaving for Bath. She finds the situation with General Tilney more entertaining than distressing. And she manages to make Henry's visit to the Morlands' even more awkward by being oblivious to his desire to be alone with Catherine – sounds like a pretty normal, pesky younger sister overall.