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If your mother-in-law breathes, she can be reasonably identified as a thot, and thots are despicable
Or so is the logic I have come across in different contexts
As a serious answer, when she harms your wife (or husband), children, or family. There's a difference between being a somewhat annoying family member, and a somonee worthy of hatred. Anything less than actual harm isn't worth the conflict and damage.