E.F.
I would do what you are doing and get your MIL a fan:) to drowned out the noise. you have the right idea, don't get her up at that time or you will be breaking habits later on. What ever you do, just be consistent and she will return to her good sleeping habits in a few weeks. It also might be helpful to talk with your MIL and explain the situation, and see if she could handle it for a few days to a week. By then she should be back to her normal sleeping habits.
Good luck,
E.