i understand your concern, but think about it - your in-laws want to do this as they really love your baby and want to spend as much time as possible with their grandkid. isn't that lovely? :)
unless they are making your child sleep really late or causing too much exhaustion and disruption to her, i think you can let them take care of this for the time they are around - after all, it's only for a month. once they leave, you will need about two weeks to get your child back in routine.
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