We have been having regular sleepovers with both boys from about 3 months. They were both bottle fed and I was very comfortable. Both sets of grandparents have full kids rooms with cots set up.

To us, the boys having a strong relationship with their grandparents is very important. They are as comfortable at the grandparents places as they are at home.

However, you will need to reach an agreement with your DH. This is something that has the potential to become a very big issue, especially if your MIL doesn't let it drop.

Good luck.