I've been wondering what their part in the story is- apart from giving John a sister to drive when Isabella sensed a proposal was imminent.

Anne and Maria appear as examples of girls following popular culture mindlessly. John and Isabella ignore their younger sisters. To try and get attention with family, they imitate Isabella's fashion idea and mannerisms.
Anne Thorpe loiters 'between two of the sweetest girls in the world who had been her dear friends all the morning'.
Maria uses phrases similar to Isabella, the most delightful scheme in the world.
I don't know if they read gothics - if asked, they'd probably exclaim they adore Mrs Radcliffe.
They don't assert their own personalities or bond as sisters. These girls are dull and pallid.
Isabella is a product of pop culture, but more commpetitive than her sisters so receives the attention she demands.
JA also enjoyed popular culture- reading modern novels, dances, fashion yet she retained her opinions and identity.

I don't know if JA used the Thorpes to point out to parents the dangers of spoiling one child or favouriting one girl at the expense of other sisters.
Mrs Thorpe imparts no commonsense or discretion to her children.
A contrast to Mrs Morland.

* After the knock at the door and entry of Henry to effect Catherine's cure, instead of taking the essay on Homespun values back upstairs, perhaps she should've mailed it to Mrs Thorpe c/o
Putney. ;)
Isabella seems to closely fit the example of the older girl in her habits.