Perhaps she pitied and esteemed him the more because he was slighted by Willoughby and Marianne, who, prejudiced against him for being neither lively nor young, seemed resolved to undervalue his merits.

"Brandon is just the kind of man," said Willoughby one day, when they were talking of him together, "whom every body speaks well of, and nobody cares about; whom all are delighted to see, and nobody remembers to talk to."

"That is exactly what I think of him," cried Marianne. (Chapter 10)

and I do blame Willoughby for Marianne’s more vehement and unjust attacks on Brandon. Although this, I believe, is the first time we see a conservation like the above in the text, it is not the first time Elinor has been privy to one—Willoughby and Marianne are prejudiced against the Dear Colonel and she pitied and esteemed him the more because of it. I agree with you and Delories that Marianne has to accept blame for what comes out of her mouth but IMO she is decided influenced by Willoughby; she is influenced both to act and speak more wildly and with less propriety than she ought from the moment she meets him.

Colonel Brandon, the friend of Sir John, seemed no more adapted by resemblance of manner to be his friend, than Lady Middleton was to be his wife, or Mrs. Jennings to be Lady Middleton's mother. He was silent and grave. His appearance, however, was not unpleasing, in spite of his being in the opinion of Marianne and Margaret an absolute old bachelor, for he was on the wrong side of five-and-thirty; but though his face was not handsome his countenance was sensible, and his address was particularly gentlemanlike. (Chapter 7)

Before Willoughby’s arrival Marianne thought Col Brandon was an “old bachelor” and she then paid no attention to him except when Mrs. Jennings’ prediction of a match between them arises. “Old bachelor” is not a nice observation but it is rooted in her wrong-headed point of view—he is older, perhaps appearing “old” to Marianne’s 16 or 17 year old eyes and he is certainly a bachelor.

The immediate advantage to herself was by no means inconsiderable, for it supplied her with endless jokes against them both. At the Park she laughed at the colonel, and in the cottage at Marianne. To the former her raillery was probably, as far as it regarded only himself, perfectly indifferent; but to the latter it was at first incomprehensible; and when its object was understood, she hardly knew whether most to laugh at its absurdity, or censure its impertinence, for she considered it as an unfeeling reflection on the colonel's advanced years, and on his forlorn condition as an old bachelor. (Chapter 8)

In Chapter 8 Marianne considered Mrs. Jennings prediction of a match between her and Col Brandon “as an unfeeling reflection on the colonel's advanced years, and on his forlorn condition as an old bachelor.” This is a rather sensitive observation on how she believes Mrs. Jennings is treating Brandon unfeelingly and IMO more in tune with her nature than the mean comments she makes after she becomes involved with Willoughby. Compare this to how Marianne acts in Chapter 12 during the Letter F conservation or when she feels Elinor is being neglected by her in-laws and takes up the screens “to admire them herself as they ought to be admired” in Chapter 34. I think Marianne’s ability to see the Dear Colonel as the victim of Mrs. Jennings unfeeling predictions of matrimony shows that she did not see Brandon as despisable before Willoughby came on the scene and makes me wonder if Mrs. Jennings predictions were really so outlandish if Willoughby had never captured Marianne’s Special Attention. (;D)

Elinor then heard Willoughby say in a low voice to Marianne, "There are some people who cannot bear a party of pleasure. Brandon is one of them. He was afraid of catching cold, I dare say, and invented this trick for getting out of it. I would lay fifty guineas the letter was of his own writing."

"I have no doubt of it," replied Marianne. (Chapter 13)