I wanted to bring this up in the discussion of last week's chapters, but I decided to wait until today, because of the first chapter in this week's reading.
Back to the question about what is the point of Margaret in the story? I agree that she was likely a correspondent in the earlier epistolary form of the novel, and we also discussed how she affects their financial situation.
Another big role that Margaret plays is that she makes Marianne a middle child. Because of Margaret, Marianne sometimes behaves more childishly than I think she would if there was only Marianne and Elinor in the family.
In Chapter 9, we see Marianne and Margaret enjoying the outdoors as two young girls.
They gaily ascended the downs, rejoicing in their own penetration at every glimpse of blue sky: and when they caught in their faces the animating gales of an high south-westerly wind, they pitied the fears which had prevented their mother and Elinor from sharing such delightful sensations.
One consolation however remained for them, to which the exigence of the moment gave more than usual propriety; it was that of running with all possible speed down the steep side of the hill which led immediately to their garden gate.
This type of behaviour is quite natural, especially for two teenagers, but somehow I can't see Elinor delighting in the wind blowing in her face and running as fast as she could down a hillside. I also can't picture Marianne doing this alone.
So, literally, if she and Margaret had not been outside 'playing', as children do, she might not have met Willoughby--or at least would not have met him then and there and in such a romantic way of having him sweep her up into his arms.
We also catch other glimpses of Marianne behaving more like a child with Margaret in this exchange from (Ch. 12)
Margaret's sagacity was not always displayed in a way so satisfactory to her sister. When Mrs. Jennings attacked her one evening at the Park, to give the name of the young man who was Elinor's particular favourite, which had been long a matter of great curiosity to her, Margaret answered by looking at her sister, and saying, "I must not tell, may I, Elinor?"
This of course made everybody laugh; and Elinor tried to laugh too. But the effort was painful. She was convinced that Margaret had fixed on a person, whose name she could not bear with composure to become a standing joke with Mrs. Jennings. Marianne felt for her most sincerely; but she did more harm than good to the cause, by turning very red, and saying in an angry manner to Margaret, --
"Remember that whatever your conjectures may be, you have no right to repeat them."
"I never had any conjectures about it," replied Margaret; "it was you who told me of it yourself."
This increased the mirth of the company, and Margaret was eagerly pressed to say something more.
"Oh! pray, Miss Margaret, let us know all about it," said Mrs. Jennings. "What is the gentleman's name?"
"I must not tell, ma'am. But I know very well what it is; and I know where he is too."
"Yes, yes, we can guess where he is; at his own house at Norland to be sure. He is the curate of the parish I dare say."
"No, that he is not. He is of no profession at all."
"Margaret," said Marianne, with great warmth, "you know that all this is an invention of your own, and that there is no such person in existence."
"Well, then, he is lately dead, Marianne, for I am sure there was such a man once, and his name begins with an F."
Obviously the two sisters had been enjoying gossiping about their older sister's love life. I can just picture them whispering and giggling about it together. If not for Margaret, this never would have come out in conversation like this in that company.