Either a mother in-law or father in-law who is meddlesome and purposefully lives to aggravate his son's wife or daughter's husband.
Rude, and oblivious humans who speak before they think about what they are about to say. They are ciritical of the cooking made by their son's wife, or critical of the income earned by their daughter's husband.
If there are step children involved, they usually remain friends with their children's exes, and spoil the step kids to the point that the little bastards become spoiled brats who are rude to the new step parent and destroy their property.
The worst undeserved punishment life has for innocent people with good intentions.
HUSBAND: My mother has offered to buy my ex a house for my (spoiled) daughter, because she is being picked on (imagined) by her cousins while she lives at her aunts house.
NEW WIFE:(after being nice to mother in law) Wait, you guys are not even together anymore, wtf?
HUSBAND: She is concerned about her grand daughter.
NEW WIFE: Was she concerned when your ex was cheating on you and got pregnant by the other guy? I HATE MY Monster In-law.