True, but think about Mary in P&P talking about how fragile a woman's reputation was. Caroline was insinuating Grace was behaving improperly. With as much time as Grace was spending in Darcy's presence (will be talked about more in the next chapter), and how sought after Darcy was as a rich and handsome man, all it would take is a few comments from women, or their mothers, who want Grace out of the way to validate the claims. Lady Dobbs and Lady Sheldon have both known Grace for a long time and care about her, they wouldn't want to take a chance that everyone would automatically ignore Miss Bingley's claims.