Isabella's break-up with James

I haven't read the book in a while, but I think it may have been partly because she, like her brother, thought the Morlands had more money than they did. Remember, John told General Tilney that Catherine was to inherit the Allens' money, which wasn't true.

I've always imagined that James raised the subject of a split because he believed that Isabella's feelings had changed and that she'd transferred her affection to Frederick, not realising that Isabella has no feelings for aanyone except Isabella. (Edward gave Lucy the option of ending their engagement when Mrs Ferrars had cut him off. She chose not to take it until a definite better offer came along.) This whole situation must have been very painful for James and I can't imagine he's the kind of man who'd force a woman who didn't care for him to marry him. Since legally, the woman was the only party who could withdraw her consent to an engagement without being sued, Isabella must have consented to ending the engagement when the subject was raised. James thought he was releasing Isabella to be happy with the man she now loved, and Isabella also thought she was being released to marry a richer man, hopefully one with a more generous father (since all the Thorpes thought the Morlands were richer than they really were). In that sense, you could say that the ending of the engagement was mutual.

Isabella broke up with James because she was keen to marry money and marry quickly. She's the beauty of the family and I think the expectation was that she'd marry well and raise the family fortunes. I think she grossly overestimated the power of her beauty. Having set her cap successfully at James, she was misled by Frederick's manners enough to think that he'd also fallen for her and was on the point of making her an offer. He wouldn't make an offer if Isabella already had a fiance (and the engagement was well known in their circle) she would have jumped at any offer by James to release her. Isabella then realised, to her horror, that she'd misread the situation, misunderstood Frederick's feelings and she'd been left with no-one. Hence her pathetic attempt to get James back, via Catherine. She's simply a less successful, less ruthless Lucy Steele type.

I doubt Isabella would have slept with Frederick Tilney. The other Austen women who had sex outside marriage did so because their passions were stronger than their sense (Maria Rushworth, Lydia Bennet, Eliza Williams). Isabella's a cold hearted gold digger, determined to marry well. She's not as clever as she thinks she is, but IMO isn't stupid enough to risk sex outside of marriage when her face and figure are her only assets.

He looks like what happens when you punch a cow!