What logic do they think they're displaying here? To describe this 'logic' as idiocy would be incredibly offensive to idiots everywhere. This is like punishing someone who's gained weight on a diet by force-feeding them cake.
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In any enterprise - business, military, academic, sports, etc. - leadership gets the behavior that they reward. Being a parent is a leadership position, and these parents have taught their other kids that acting like the middle sister is the behavior that they reward. They can't be surprised that their other kids have learned the lesson they've taught.