You say your in-laws are nice people so I believe they will understand if you put your point across to them.
A little shift in routine is okay because they are there for only a month and it is bound to happen that they would like to spend maximum time with their grandchild. Plus with them spending a lot of time with your daughter you get a little more time for yourself and your husband!! Make the most of it :)
You shouldn't worry too much.
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