I think Fanny disagrees with Mary’s sentiments on enforced religious observance, I mean Fanny obviously thinks regular devotion is a good thing but if that disagreement was all to upset her I think pity would be her primary emotion. Fanny would feel them wrong but I don’t think she would be angered at the idea of reluctant, unsteady bells whose prayers are interrupted by worldly concerns. She probably sees Mary as such a one after her description. Mary also laughs at Fanny’s feeling it is a fine thing for a family to assemble regularly for prayer but it seems to me she has warmer feelings and takes more offense on behalf of those she loves than anything else:

For a few moments she was unanswered. Fanny coloured and looked at Edmund, but felt too angry for speech… (9)

I feel what probably makes Fanny angry is Mary’s attack on past and present parsons—especially present soon to be parsons: ‘if the poor chaplain were not worth looking at—and, in those days, I fancy parsons were very inferior even to what they are now’ (9). Fanny’s looking at her cousin makes me feel her anger is at Mary’s (unintentional) criticism of Edmund’s worth. This quick warmness on the behalf of a loved one is not a new reaction. When Mary criticizes men in general and brothers specifically for not writing something worth reading, especially to their sisters, Fanny colors for William’s sake (6) and later queries Edmund:

“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?” (7)

At Sotherton (10) Fanny is surprised to be left “to her solitude” but stronger emotions do not take hold until Henry entices Maria to go into the wilderness with him. She is “astonished at Miss Bertram” but “angry with Mr. Crawford” so this time Fanny’s anger is riled by a threat to someone she cares about. On the other side of this equation Edmund was vexed with his mother and aunt’s ill-use of Fanny but he was “still more angry with himself. His own forgetfulness of her was worse than anything which they had done” (7). Later he is made “too angry to speak” (15) after Mrs. Norris mercilessly condemns Fanny for refusing to act.