Agree with Evelyn. Speak to them and explain to them that your baby's routine has been established and there should be minimal disruptions to that schedule.
I think they should be able to understand. Also try to highlight that it will be very difficult to re-establish new routines every month (since they will be staying with you for a month).
During their stay, you can enlist their help whenever possible (while they are not napping) so that they will get to spend time with your daughter (instead of having her stay up at night). All the best!
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