Elizabeth's remark about Lady Russell that "she sits so upright" is an interesting commentary on the fashionable style current c.1812-1814.

As Elizabeth implies, the fashionable posture at that time was not upright, but altogether more relaxed - best described as a sort of graceful slump. The page linked at the foot of this post has numerous fashion plates from 1812 issues of 'Ackermann's Repository', the fashion magazine. The seated ladies illustrate this posture very clearly.

Now check out this image of two ladies seated outdoors from Heideloff's 'Gallery of Fashion' September 1794. They sit so upright!

http://locutus.ucr.edu/~cathy/heid/ND9-1794.html

We don't know Lady Russell's exact age, but these are the fashions of her youth, before the high waists, narrow silhouette and soft flowing lines imitating classical drapery came into vogue.

Women's underwear changed too. Until the classical look took hold in the late 1790s, fashionable women wore relatively long stays with a stiff busk down the centre-front which would have made a slumped posture impossible. When the fashions changed, women either abandoned their stays altogether or took to wearing much shorter stays which functioned rather like a push-up bra.