I think it was her love of money. She fell in love with Edward and saw a great future as grand lady (only being the niece of a teacher). But, after years of engagement, when their relationship finally comes out, Edward is disinherite. So, she can make a big black cross over her future as grand lady and will become 'just' a clergyman's wife (=have a lot less money than anticipated). But, the brother, who is now the heir, has an eye on her and she decides to switch brothers and secure her future as grand lady...
I think that was it. She was a little bit of a selfish nasty cow. A little like her sister-in-law, actually.