Here's the thing: you will never persuade your mother-in-law or your husband to see your point of view. You have to begin acting more authoritatively, without being hostile. When your mother-in-law complains, don't let your self get hooked into the angry energy. Just stay calm and say, "Hmm." Then do exactly what you think is right for you and your child, no matter what kind of fuss she makes. When your husband is around, if your mother-in-law monopolizes him, either do something you enjoy by yourself, or draw a clear line by saying, "I'd like to spend two (or three, or four, or whatever) hours alone with you today. If you want to spend them with your mom, it will damage my feelings for you."
This may not always work out the way you ant it to. But by being very clear about your needs, you help define the future relationship you'll have with your husband and his mother. Most likely, they'll resist you briefly, but if you're clear and calm, without trying to force them to agree with you, they'll make adjustments that will improve your whole family dynamic.