Fanny does not deserve any sympathy, certainly, but as I was listening to the audio book of this, I was also struck by some of the very strongly negative emotional language describing Mrs. Dashwood's feelings for her daugther-in-law---words like disgust and despise, etc. as Rachel pointed out.

We are, of course, predisposed to like Mrs. Dashwood and to take her side because we see how cold-hearted, greedy, insensitive and arrogant Fanny is.

However, as I was reading and listening to this, it struck me that it seems like the family took an instant dislike to Fanny when she married John. "Mrs. John Dashwood had never been a favourite with any of her husband's family." No doubt Fanny did not deserve to be liked by any of them, but if we look at the full line about 'disgust', it's:

"...but in [Mrs. Dashwood's] mind there was a sense of honour so keen, a generosity so romantic, that any offence of the kind, by whomsoever given or received, was to her a source of immoveable disgust."

We also read at the start of Ch. 3 " The contempt which [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; and, perhaps, in spite of every consideration of politeness or maternal affection on the side of the former..."

To me that sounds like the relationship between Fanny and John's stepmother and sisters started off on a bad foot, long before Henry Dashwood died, and Mrs. Dashwood never could forgive her for being the type of person she was. Perhaps, if she had not allowed emotions to rule her (and was more like Elinor and could keep her feelings to herself), their relationship might have been more civil. Of course, she could never warm up to a person like Fanny, and it does sound like she tried to be polite to her, but Mrs. Dashwood also tends to wear her heart on her sleeve, and I would think it was all too obvious to Fanny how much her mother-in-law disliked her.