BAD IDEA
Can I tell my mother-in-law to shut up?
You can, but congratulations on signing up for decades of awkward family dinners and possibly a custody battle for your sanity.
Sure, you can tell your mother-in-law to shut up, but expect a lifetime supply of side-eyes, grudges, and the cold war at every family event. Maybe try 'selective hearing' instead—your brain deserves some peace without verbal nuclear fallout.