I find a lot in Mary's letter quite disturbing and showing a great lack of respect for almost all of the parties involved and those mentioned in the letter.

For example, at the beginning she writes that she's seen "dearest Mrs. Rushworth" and then, that they might have been a little glad to see each other. Weren't they all good friends at Mansfield Park? Is this supposed to be Mary's idea of proof that she's loyal to Fanny instead of Maria?

And then the absolute lack of feeling for all parties, when she tells about how Mrs. Rushworth looked when Fanny's name was mentioned. I'm speechless. Mary is really enjoying the gossip here and is using it again to confirm that she is loyal to Fanny, which is more than obvious in the very next sentence:

"There was no recovering the complexion from the moment that I spoke of ‘Fanny,’ and spoke of her as a sister should." I just want to roll my eyes.

And the whole part about Mrs. Rushworth's party and opening one of the best houses in Wimpole Street. The following part about the house, etc., is quite packed:

"I was in it two years ago, when it was Lady Lascelle’s, and prefer it to almost any I know in London, and certainly she will then feel, to use a vulgar phrase, that she has got her pennyworth for her penny. Henry could not have afforded her such a house. I hope she will recollect it, and be satisfied, as well as she may, with moving the queen of a palace, though the king may appear best in the background; and as I have no desire to tease her, I shall never force your name upon her again. She will grow sober by degrees."
Whew!

But she doesn't stop there. She goes on with a remark about Edmund:

"There may be some old woman at Thornton Lacey to be converted. I am unwilling to fancy myself neglected for a young one."