Like her other novels, Austen points out that the expectation of women is to marry well. In other words, middle class and genteel women were expected to marry men who were independently wealthy and could hopefully offer an advance in social status. It is expected that Maria, Fanny's cousin should marry well. In fact it became her "moral obligation, her evident duty to marry Mr. Rushworth" who had a "larger income than her father's" (Ch. 4). Maria becomes engaged to him very soon.
However, because Fanny is the daughter of poor relations and is being looked after out of charity, expectations of Fanny are different. Puzzled by Fanny, Miss Crawford asks Edmund if Fanny has yet been introduced into society. She sees that Fanny is the correct age, but she also sees that Fanny says so little that it seems she has not yet been introduced into society. It is Fanny's opinion that she is quiet because she is not asked her opinion. Miss Crawford finally determines that since Fanny has never been to a ball, she has not yet been introduced into society by her family (Ch. 5). Hence, it is expected of Fanny to just continue existing without any hope of attaining marriage or social status.
However, things begin to change for Fanny when Sir Thomas returns. When Fanny is invited to dine at the Grants, Sir Thomas even requests the carriage for her, arguing against Mrs. Norris's claims that Fanny can walk in the rain (Ch. 21). While Mrs. Norris still wants to consider Fanny beneath their family, Sir Thomas's offer of the carriage serves to treat her as a lady. Sir Thomas even begins to see Henry Crawford as a good match for Fanny, which would increase her wealth and social status (Ch. 25). Hence, after a while, even Fanny is expected to marry for financial benefit and increased social status.