...yes, she is making herself a little too comfortable in a home not her own, but Miss Bingley is single, and according to Jane, she has come to keep house for her brother. To us, she may not be doing very much, but Amanda Vickery, in "The Gentleman's Daughter", mentions that bachelors and widowers at the time were generally overjoyed to get a sister to run their houses for them, because of the strict division of labour between the sexes (sisters-in-law, too, because men were not supposed to marry a dead wife's sister). Also, IMO, Bingley would be considered just as responsible for Caroline as Mrs. Hurst, so Caroline does have a right to be there. Darcy may or may not like it, but Miss Bingley is a fait accompli!