I like your point it makes perfect sense to be offended by Fanny’s insensitivity. This time I got a stronger sense of distance and dislike between the Henry Dashwood and John Dashwood families than I have in the past—Mrs. Dashwood’s sensibilities with regard to her daughter in-law especially. The immoveable disgust of Mrs. Dashwood and “so earnestly did she despise her daughter-in-law Elinor had to “strive to rounse” Mrs. Dashwood to receive Fanny with proper attention and forbearance. Fanny’s behavior deserves no defense but I also noticed she was disliked even before her decent on Norland:

The contempt which she [Mrs. Dashwood] had, very early in their acquaintance, felt for her daughter-in-law, was very much increased by the farther knowledge of her character, which half a year's residence in her family afforded… (3)

It goes both ways I think. In Ch. 2 Fanny points out to her husband:

Your father thought only of them. And I must say this: that you owe no particular gratitude to him, nor attention to his wishes, for we very well know that if he could, he would have left almost everything in the world to them." (2)

I do not know if Henry Dashwood wished to exclude his son completely as Fanny suggests but he “had wished for it [Norland] more for the sake of his wife and daughters than for himself or his son…” per Ch. 1. The greedy John Dashwoods could hardly have been content with such a belief but his father does appear to have favored his second family—they were “most dear” to him per Ch. 1. With such a son and daughter who can blame him? Perhaps that is why John, Fanny and little Harry only visited occasionally.

Thanks for reading! (:D)