The part in which Fanny persuades her husband to for go his promise to provide for the Dashwood sisters, what does this conversation reveal about their characters and why would the author spend so much time on this conversation?
Well.. Yes, chapter ll is wholly focused on Fanny's persuasion to his husband to reduce the money he has decided to give to his half- sisters.
Answering your first question, both the characters are unwailed to a greater depth. John Dashwood seems a vacilating character, he seems not having stable opinion and is easily manipulatd. Fanny, in contrast is manipulaive. She is cunny, at first she proclaims it is thier son's wealth.
How could he answer it to himself, to rob his child, and his only child too, of so large a sum?
she is very manipulative. Fanny's jealousy is revealed by her arguments. For example she says thier father would not have been in his right senses when taking the promise. Fanny insists on the treatment of the others towards the relations of half- blood. Next she claims her bad experience of getting used to pay annuities, therefore John doen't want to get into trouble and gives up. Three thousand poubds reduce upto nothing. Fanny's jealousy, manipulative and cunning behaviour is evidently portrayed.
Austen has taken a chapter to show this episode. This episode reveals the method of inheritence and importance of wealth which itself is a theme in the novel. Austen was very talented at creating vibrant characters, her excellency is portayed by the demonstration of Fanny's behaviour to the construction of Fanny's character. John Dshwood's weak personality is also taken to light. Spending much time on this episode has only enhances the argumentative nature of Fanny, to which finally her husband agrees. She has brought a lot of examples to prove her facts, her greediness is further highlighted by thier long conversation. I think Jane Austen powerful portraiture of the two characters has lenghthen the episode.