This is an expansion of a thought I posted on Austenuation as part of a comparison among novels, but wanted to explore here in the sole context of NA as well.

Isabella uses flattery to attach Catherine Moreland, we all know that now. But when things don't go her way we get a glimpse of her true character. A telling scene is in Chapter 13 when Isabella tries to flatter and cajole CM into joining them all on the second buggy ride. When CM demurs due to her preexisting engagement with the Tilneys, Isabella's feelings are crystal clear: ... a short silence ensued, which was broken by Isabella, who in a voice of cold resentment said, “Very well, then there is an end of the party. If Catherine does not go, I cannot. I cannot be the only woman. I would not, upon any account in the world, do so improper a thing.” She's fickle toward CM, but still tries to redirect everyone's attention to her supposed sense of propriety; I doubt that was really uppermost in her mind based on the narrator's use of the phrase "voice of cold resentment".

We see the doubletalk again in Chapter 16 when she receives the letter from James regarding the support he will receive from his father, although here I think it is even less disguised despite her best efforts (and those of her mother).

“Nobody can think better of Mr. Morland than I do, I am sure. But everybody has their failing, you know, and everybody has a right to do what they like with their own money.” Catherine was hurt by these insinuations. “I am very sure,” said she, “that my father has promised to do as much as he can afford.”

Isabella recollected herself. “As to that, my sweet Catherine, there cannot be a doubt, and you know me well enough to be sure that a much smaller income would satisfy me. It is not the want of more money that makes me just at present a little out of spirits; I hate money; and if our union could take place now upon only fifty pounds a year, I should not have a wish unsatisfied. Ah! my Catherine, you have found me out. There’s the sting. The long, long, endless two years and half that are to pass before your brother can hold the living.”

“Yes, yes, my darling Isabella,” said Mrs. Thorpe, “we perfectly see into your heart. You have no disguise. We perfectly understand the present vexation; and everybody must love you the better for such a noble honest affection.”

Amazing! Her own mother is encouraging her in the subterfuge (albeit perhaps unintentionally), yet seems to miss the fact that she is not really promoting her daughter's happiness. As someone noted below (regarding the Chapter 15 family dynamics resulting in the younger Thorpes' "undefined discovery" of the engagement before it was announced), it's no wonder the children behave as they do considering the example their mother set for them.