When did you all let your baby stay the night out?

My mother in law keeps pressuring me to let my daughter stay the night at her house.  She is only 3 months old and I feel that that is WAY too young.  My sister in law let her watch her son 2-3 time a week over night starting at 3 weeks old.  I can't do that.  Any advise on how to get her to back off?  

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