It always astounds me how shocked people are when they had an affair with somebody, that person left their spouse for them, and then, years later, that person leaves them. She already knew he was capable of walking out on a wife and two girls on the brink of adolescence just so he could have more playtime with you, and then she's shocked that he does it to her.
And I'm with the others, she was hugely responsible for the relationship and breakup to begin with (who offers her house as an overnight to a married man on a regular basis without intending something?), and then it seems like she expects teenagers to immediately accept her. They're teens, they've got all sorts of hormones going on, and moreover they are just at the age to be thinking about relationships and what it means to be a responsible adult, and what an example they get. And *then* when they don't immediately adore her and gush over all the work she's done, she throws a kid's plate out the window? That was probably the moment where she broke it. Maybe they might have eventually come around, MAYBE. But right then and there, she established herself as somebody who cared more about them doing what she wanted and being the perfect happy lovely family (which *they* probably thought they had and saw her as the interloper in) than about them.