MIL wants grandkids one weekend per month…

I overhear my MIL and BIL scheduling months in advance about which weekends his kids are going to spend the night. She also complains she just can’t go 30 days without seeing them, that she should be the one called to babysit on weekdays, not anyone else. My daughter is two and I don’t plan on having her spend the night anywhere for a long time, let alone a whole weekend. She still wakes up and wants me, on top of all of that has food allergies to common foods and I just don’t trust anyone. Weekends are our family time and I don’t want to give up one every month. I am pretty sure my husband would agree, but we haven’t talked about it yet because she hasn’t asked. How would you approach this with her when the time comes? Why is it so hard for me to share my baby. Also I think it’s the fact that she demands it like a custody agreement 🙄