Accept that your in-laws, with all of their flaws, are part of your life. “If you plan on sticking with your spouse, then you're also stuck with your in-laws.” Complaining that they are the way they are, demanding that they change and/or that your spouse make them change, is going to be about as effective as demanding that your brother stop teasing you or your own mother stop asking you when you are going to color your roots. This doesn’t mean that you actually have to like your in-laws. But it does mean that they are part of your life and that it would be good to find some ways to live in peace with them.
It may be as simple as accepting that your mother-in-law is going to feed your child ice cream, no matter how much you protest. (This was one of the places my own mother-in-law and I came into conflict. Her favorite line was “It won’t hurt him just this one time.” But of course, I knew that we weren’t talking about one time.) There is no good solution to this set-up. If you demand that your child not eat it, you become the mean and restrictive one in your child’s eyes. And you turn ice cream into a “forbidden fruit,” which makes it all the more enticing. And you give your mother-in-law and everyone else who feels like making a big deal out of it fuel for criticizing you as an overprotective, neurotic mother. For sure it’s a way that she has of not respecting you, but perhaps your best response is the high ground. You will come away with more self-respect, and your child will not die or develop high cholesterol or an eating disorder from the ice cream. (If there really is a life-threatening issue going on, and your mother-in-law is flouting a medical necessity, then it’s a very different issue.) Over time, I actually came to agree with my mother-in-law – one time was not going to make a huge difference in the course of my Daugther’s life. And interestingly, since she discovered that she didn’t particularly like ice cream, it turned out to be a positive experience that he got to taste it with her.