MIL took my newborn out while I was sleeping. Call her and she said bring the bb out for her xmas gathering. Gone for few hours now and she didn't even bring diapers and milk powder. How do I tell her not to do that again?! This is really frustrating!

You might want to try telling her nicely your concerns and reasons why she should not do so, like the Xmas gathering might have many people around and you might not know who's sick or who's not and the baby might fall sick, It might be tough to expect our MIL to listen and follow everything but setting boundaries might help in this situation and it will lessen the tension.