L.S.
OK. I'm a little older ... 45 to be exact, so I have a little different opinion about this. When I was a new mom, I felt exactly the same way as you do now. My in-laws lived nearby and they babysat a lot. They did lots of things that irritated me. I didn't like the way my MIL held my sons, for instance. She never held them close to her body, but kind of laid them in her lap and put her hands behind their heads. She gave them foods I didn't give them, they let them watch more t.v. than I did, put them down for naps at different times and in different ways than I did. And my father-in-law gave them bubble gum all the time, even when they were 2 years old!
So here's my take on it now that my sons are nearly 19 and 17 years old. When I was a new mom, my parents' generation had their own ideas about raising kids. They gave us cereal when we were 2 months old, held us in their laps when they drove us places, and never heard of a bike helmet. Times changed, we learned more about safety and nutrition, and we did things differently. Well, the same pattern continues. When I was raising my kids, we had no problems with giving babies juice at 10 months. I'm not saying it's a good thing to give babies juice. But, perhaps your mother-in-law just remembers how she did things and knows that her kids turned out fine.
I'd also say, don't freak about so much about things that your in-laws do that are different. Certainly do stand up for really big important things, like car seats and smoking. But when it comes to the things they feed your baby, just be patient and explain what you've learned. And know that she will probably say something like, "Well, we never did it that way." And also know, that if they do some things different from you, don't fret about it so much. They may not know your child the way you do, know all of your baby's likes and dislikes, but they love their grandchild very much. If you can relax a little, you can give your child a very special gift by fostering a wonderful relationship with her grandparents.
You know how I said my father-in-law gave my boys bubble gum all the time? Well, I was not at all pleased about that when they were 2 (and I still would not be pleased about bubble gum for a toddler). He died just this past November. My boys loved their pop-pop and they had the great blessing of being with him in the hospital at the very moment that he died. At his funeral, the boys placed some bubble gum in Pop-pop's casket before it was closed. That's when I realized that all those times that I fretted over their dental health just didn't compare to the relationship with their grandfather. To them, Pop-pop always having gum in his pocket for them, even when they were teenagers, was a special touchpoint in their relationship with him.
So try to be patient over the little things. Save the battles for things that really come down to safety and life-threatening health issues. And give your daughter the gift of her grandparents.