Mother-in-law/monster-in-law jokes exist for a reason. They poke fun at the complicated and, at worst, strained relationships many people seem to have with the mothers of their spouses.
This story is likely one of the many you’ve read on this site. A woman visiting her daughter decided to step on her son-in-law’s boundaries. Her actions were way out of line and upset her grandson, yet she refused to apologize.
Unfortunately, the chaos strained the man’s marriage, which made him wonder if he may have overreacted.
