It's not your responsibility to "bear the brunt" to protect other members of your family. The rest of your family are adults, capable of making their own decisions, not hostages. You are not responsible for anyone but your little family unit.
What really helped me set boundaries with my narc DM was talking through with DH how I wanted DS's childhood to be (nothing like my own)!. Really talking about our hopes and ideals for bringing up DS helped me get very clear on what I would and would not tolerate from DM. It also means we have a mostly united front on dealing with her, whereas before she was a huge source of conflict in our relationship.
The best way I can explain how to set boundaries with a narc is like training a dog. The punishment has to happen instantly the boundary transgression is made. So any dig or sarky comment no matter how small, I say "pardon? What do you mean? Did you just tell me how to butter toast? I'm 32 years old, stop telling me how to butter toast." And then change the subject. It is scary at first but you will start to enjoy standing up to her and feel relieved!
Ultimately a narc uses the rules of social convention to get what they want. DM knows you won't want to make a scene. So she is in control because she will stop at nothing to get what she wants, no matter how crazy she looks doing it. Same with insults and sarky comments. She knows you will try to be polite and avoid the conflict.
You can learn to recognise when you are letting her get away with behaviour that you would not tolerate from any other human being on the face of the earth. You can learn how to calmly enforce a boundary in the moment she tries to transgress. You just must be willing to get very real with yourself and abandon any thoughts of reconciliation. You didn't break her, you cannot fix her. Your priority has to be yourself, your DH and your DC.