Like others have said, it's best to not have any expectations when it comes to your MIL. How she choses to act is on her. What's important is that you and your wife are on the same page and provide a united front.
I am a strong, opinionated, sometimes stubborn person too, so I respect that, but the difference is that I don't force my opinions and thoughts on others and get pissed when they don't follow suit. Like my ex-MIL. On it's own, people like this would be a nuisance to a marriage; for me, the deal-breaker was that my ex totally bought into anything his mom, wanted, demanded, etc. Everything. It was ridiculous. He would agree with me on things, then his mom would get involved and he would switch to her whatever she said. Worst thing about it is that she knew he would do this, and use it to manipulate situations. She actually admitted it to me once, saying "Don't be mad at him, you know he'll do whatever I say or ask him to do". She was working full-time (a nurse) but yet would get him to come over daily to brng her paper and mail to the house (25 ft from the door), do her chores, mow her lawn, vacuum her house, bring her food, get groceries, etc because she was so "tired, sore, fill in the blank". When I was pregnant I said to her that I was so sorry she wouldn't be able to watch the baby by herself, and she said why not. I said, well, because if your health won't allow you to get your mail or do basic household chores, it doesn't seem you would be able to take care of a baby/toddler. Her health miraculously improved for a few years. I used to joke that my ex-MIL was the "other women" in my marriage to her son. ((Little did I know she was one of about five, but that's a story for another thread haha)).