Fanny is totally a snob and a half. She basically only wants to associate with people richer or more important than she is. We think that it's really lame of her to constantly be dissing the Dashwood sisters, since, after all, she married their half-brother. If John was good enough for her to marry, why shouldn't Elinor be good enough for Edward?
The answer is simple – it's all a matter of money. Happiness and love are secondary to Fanny; first things first, she wants to make sure that the Ferrars clan increases in wealth and status. She and her mother are both determined to find Edward a wife that will get them all somewhere – somewhere higher up, that is. Miss Morton, with her landed gentry background and inherited fortune, is the only suitable option for Edward in Fanny's eyes, despite the fact that he doesn't even know Miss Morton, much less love her. Fanny's constantly on the lookout for boosts either to her ego or to her progress up the social ladder (or preferably both at once), and, seeing as Elinor and Marianne offer her neither, she has no use for either of them.
Fanny does look upon her sisters-in-law once they develop worthwhile acquaintances – for example, she's willing to put up with Elinor and Marianne's company once the Middletons (who are higher up in the world than any of the Dashwoods) make an appearance. Fanny and Lady Middleton are a match made in heaven – they're both self-centered, narcissistic, and social climbing bores. Fine, we say. Frankly, they deserve each other.