If there's scream and shout ecvery now and then, very soon, your kids will learn the same. On the other hand also can't blame your mother in law....maybe that's the only way she knew. I watch the show "supermom" a yr or two back, active kids must stare them into the eye, wait for them to calm down before giving instructions, otherwise, they hear nothing and continue with their irritation or hyper. Find a method that works in front of your mother in law adn ask her to adopt that but of course you need some strategy like telling her screaming will be bad for her heart blah blah