because they were nineteen and twenty (ch. 5); at most, he could have been twin brother of one of them.

The sentence "Henrietta and Louisa, young ladies of nineteen and twenty" seems to mean that Henrietta was 19 and Louisa 20; but in chapter 9 Mary is discussing with Charles about the possibility of Henrietta marrying Wentworth, and she says that Charles Hayter is "a most improper match for Miss Musgrove, of Uppercross", which could indicate that Henrietta is the eldest sister.