Agree, I thought Mrs. Dashwood was particularly ungenerous and mean to Elinor in Chapter 15—much more than Marianne when she accused Elinor of being cold in her description of her feelings for Edward in Chapter 4. Mrs. Dashwood's affectionate, kind-hearted nature and her affection for Elinor seem overwhelmed by her disappointment in Willoughby's strange and unexpected departure; she cannot bear the idea that Willoughby is going to fail them. Although I too must wonder if she actually knows her daughter I think this attack on Elinor involves more that just the misunderstandings between two dissimilar characters. I think the vehemence of Mrs. Dashwood’s questioning accusations is because an emotional outburst has been building on each subsequent disappointment she has experienced since the opening of the novel.

I think Elinor’s suspicion of Willoughby is the last straw for Mrs. Dashwood in a long line of broken straws that results in a sort of tantrum where Elinor is obliged to defend herself against her mother’s head-in-the-sand attitude. First, her husband’s inheritance was a disappointment and before he could enhance their security by saving from his increased income he dies leaving them few options. John Dashwood does exactly as he opines his father could not have believed possible of him—he neglects his step-family. Next she believes a marriage between Elinor and Edward is a sure thing—this would not only secure Elinor’s future but also the rest of her little women. Edward if he had means would never leave them in want. Then Willoughby comes along probably giving her the same sort of imagined security and now he has disappeared from their life with no word of his return.

I do not mean to say that Mrs. Dashwood is greedy but that her concern for the future security of her girls is probably greater than her outward declarations would imply. Her concern for their future is legitimate and I do think she is telling the truth when she says “men are very safe with us, let them be ever so rich” in Chapter 9; they are not gold-diggers. Men are not safe in being counted upon as sure husbands for her daughters however. Mrs. Dashwood jumps to conclusions about Elinor and Edward and then Marianne and Willoughby just from her observations, without any word of conformation so I have the sense that Mrs. Dashwood came to really count on both of the marriages taking place leading to great disappointment when no engagement was confirmed. ;D