If you want her to follow your rules, you must be very patient and explain why it's so over and over until she accepts. Video monitoring is illegal for a babysitter without noticing.
The mother needs more respect from the daughter in law than the bio mother.
Mother in law doesn't have the responsibility to take care of her grand children. If you really want to feed the kid by the book, take a parental leave and do that by yourself.
Were I the mother in the story, I won't argue with the daughter in law at all. Just order a ticket online and move to a hotel immediately until the flight due date.