Well, I see things quite a bit differently here because I think this is a surface issue, and the root issue is where the problem is.
First, I would be praying these Scriptures every day and claiming them for my husband by using his name in them:
Genesis 2:24 For this reason a man [husband's name] will leave his father and mother and be united to his wife [me], and they [we] will become one flesh.
Matthew 19:5 For this reason a man [husband's name] will leave his father and mother and be united to his wife [me] , and the two will become one flesh
Matthew 19:6 So they [we] are no longer two, but one. Therefore what God has joined together, let man not separate.
Ephesians 5:31 For this reason a man [husband's name] will leave his father and mother and be united to his wife [me], and the two will become one flesh.
Then, when and if MIL's behavior offends me enough to bring it up, I'd let my husband know about it and how it makes me feel, and then I would LEAVE IT IN HIS HANDS, knowing that YHWH is working on it because of His promises listed above that I have claimed.
I would fight this in the Spirit, NOT in the flesh. I would look for ways to bless my MIL every chance I had because we are to bless when persecuted and overcome evil with good.
What is really going on here is an attempt by satan to steal, kill, and destroy your marriage and family because he already knows what shaky ground it is on since your husband has never cleaved to you. If he had, that would be evident by his cleaving to you instead of his mother.
He is letting his feelings for his mother (and it doesn't matter whether they are positive or negative) have preeminence over his feelings for you and the children because that is what is dictating his behavior. He would rather deal with your complaints, discomfort, whatever, in regard to his mother than to stand up to her himself and make it plain that he is one flesh with you (that his primary attachment is to you, not her anymore) by letting her know HIS displeasure at her behavior toward his loved ones.
Even if he personally isn't displeased at her behavior, if he was one flesh with you, your discomfort would be enough to motivate him to do something to comfort you.