10. This MIL being described is more insufferable than mine was, but she's describing a very specific phenomenon/type of dysfunction that they share: an inferiority complex. My MIL was the product of extreme favoritism, raised as the scapegoat in contrast to her sister the Golden Child, by a controlling and hyper-critical mother. Conditioned by constant criticism and blame as a young child, she was easily triggered by the slightest idea that people were hating her or blaming her or simply favoring someone else, and once this idea was invoked she lost all ability to reason, or see evidence to the contrary. Then she'd sometimes become resentful or angry, sometimes tearful, sometimes nearly hysterical.

Once someone is triggered like this by a childhood trauma it's often impossible to reason with them or even soothe them by addressing it in any way (this is also true of my mother who has ptsd from a totally different kind of abuse). It's possible that reopening the subject later could be done if you manage to do so in a manner that prevents her inferiority complex being triggered, like with positive statements about what everyone's intentions were and apologies for the way she felt, perhaps. But it's also possible that she'll just go back through the same emotions every time she gets reminded.