Yes, I would show her routine and drill it into her how important it is to you! Also, can you get some blackout something that you can take that isn't permanent that means she doesn't have to replace curtains, etc, but that will make the room darker? Not sure your relationship with MIL but for me, if mine said something along the lines of that, it would only make me want to do it more! At the moment, you need him to nap at her place, worry about whether or not he will nap elsewhere when the time comes!