When my son was born (nearly 9 years ago), I had similar problems... not with my mother-in-law, but with my own mother! She was only 61 at the time, and was much younger that her years would have suggested. As we could not really afford daycare, and were not thrilled with the idea of putting a newborn in the care of strangers... my mother and my two sisters volunteered to care for my son at my home.
While I was grateful for the help, it quickly turned in to daily debates and an all-around nightmare! My oldest sister often deliberately did things or did not do things that I had specifically asked. My mother constantly tried giving him cereal and solid foods (banana, peanut butter, etc.) well before he 4 months old and wanted me to leave his car seat at home so she could go out during the day (because she was "bored" at my house all day). Granted, she was not elderly or senile in any way, but she didn't pay the greatest attention when driving, and was often forgetful.
I explained on several occasions that she should not be feeding him any type of cereal or solid food because he wasn't old enough and might not have been able to swallow it properly, AND it could cause food allergies. I also explained that I did not want her "toting him around town" just because she was bored... if she had errands to run or doctor appointments, I asked if she could please schedule them for days when she wasn't watching my son. She would also often forget to keep record of his bottles and diapers (ie. when and how much he ate, and when she changed him and whether the diaper was dirty or wet).
Even after explaining my reasons and repeatedly telling her that I did not want her feeding him solid foods, I continued to find evidence that she was doing it anyway. I eventually had to hide all the cereal bowls, spoons, and packages of baby food and cereal in an effort to prevent her from continuing to feed these foods before he was ready.
She also informed me at some point that since I refused to leave his car seat, she had purchased one at a yard sale so she could go out during the day. I absolutely put my foot down, since the car seat appeared to have been manufactured sometime during the early 1970's!!! I eventually gave in and began leaving my car seat, but even that wasn't good enough. She then started asking me to leave the stroller, so she could simply walk across the street to the mall... down our residential street, and across 8-lanes of traffic with no sidewalks or crosswalks of any kind. ABSOLUTELY NOT!!! I felt the need to keep the stroller in my car at all times, to prevent her from doing it anyway.
It was a constant battle with my mother, with her often "reminding me" that she had raised three children and she wasn't doing anything with him that she hadn't done with us, and that we had all turned out just fine. After months of heated discussions with my mother, and being told that I was overbearing, and that my requests and "rules" were ridiculous, I decided to enroll my son in daycare.
To end the situation, I simply "reminded" my mother AND both my sisters, that while I appreciated their offer to care for my son, I was his mother and that they should have respected my decisions and choices regarding how I wanted them to care for him. Since they were unwilling or unable to do as I had asked, I had chosen to enroll him in daycare.
To wrap up a very long story... this is YOUR child... no one should be so flippant with his heath and safety. If your family is unwilling or unable to address your concerns, they should not be entrusted to care for him.