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!

Try to communicate with your MIL to let her know your expectations on her, e.g. in this case she should never bring baby out without your presence at all. We need to be firm right from the start to let MIL understand our expectations. The key here is communication and understanding.