It wasn't just in Jane Austen's time or in England that it was considered proper for the daughters of a family to be married in order of age. In Papa's Wife by Thyra Ferré Bjorn, an autobiographical novel about a Swedish pastor's family who came to the United States at the turn of the twentieth century, the second daughter falls in love before the eldest, and when she announces it to her parents, her father is quite shocked and says that in the old country, a younger daughter would never marry before her older sister, but since they do things differently in America, he thinks it would all right as long as she asks her older sister's permission first! (I don't know if this means that if an older sister didn't marry, none of the younger ones would, either?)