I think Fanny sees so much more in people’s behavior than other folks at MP. Fanny did find amusement in Mary’s conversation about brothers and letters; she almost laughed but was brought up short by the idea a brother would not treat his sister well. In her position I can see why it would strike Fanny seriously. I think she depends on William’s affection and of course the mode of it is by letter. Edmund does try to explain Mary’s open pleasantry:

“Yes, except as to his writing her such short letters. She made me almost laugh; but I cannot rate so very highly the love or good–nature of a brother who will not give himself the trouble of writing anything worth reading to his sisters, when they are separated. I am sure William would never have used me so, under any circumstances. And what right had she to suppose that you would not write long letters when you were absent?”

“The right of a lively mind, Fanny, seizing whatever may contribute to its own amusement or that of others; perfectly allowable, when untinctured by ill–humour or roughness; and there is not a shadow of either in the countenance or manner of Miss Crawford: nothing sharp, or loud, or coarse. She is perfectly feminine, except in the instances we have been speaking of. There she cannot be justified. I am glad you saw it all as I did.” (7)