And where is your husband in all this? What is he doing to keep her off your back, to give you and his children space from his overbearing mother?
In short - is he reverting to dutiful son, and choosing not to be a supportive husband and protective father?
And if he is - what can you do to ensure he never pulls this crap on you again?
I'd suggest he's failing you because it is easier for him to do so. He is taking the path of least resistance and prioritising himself over you and DC. If he lets his mother do what she wants, he gains her approval and does not expose himself to her insults and temper - no, he delegates you to take it on his behalf [anger].
Your husband needs to behave like one, and tell his mother to show his wife some respect and courtesy or she can be thrown out of your (his and you) house and never welcomed back.
Now, at the moment, he has no incentive to step up. He keeps schtum, she has a go at you and you take it and he's free and clear. So that is a dynamic that has to change. I would suggest stage one is that you give him twice the shit that his mother is giving you, and make it clear that will only stop when he deals with his mother. There will be no peace for him unless there is peace for you. Misery loves company, and the misery she is handing out will be as nothing to the misery that you inflict on him until he reins her in. You will accept no protestations that he cannot do anything, he just has to get on with it.
Seriously. Make it such hell for him that when faced with the choice of dealing with his mother's anger or dealing with yours, dealing with his mother becomes the easier option.