@Legomum78 it seems that you have the measure of her and the negative impact this has on you. There will be many others in the family who will have questioned or been left hurt by her behaviour but not spoken out - they may never do so.
I would say that you have found YOUR stance and that you need to decide on your course of action that protects and distances you and your family and I would enact it calmly and consistently with new firm boundaries that you put in place to fade / nudge out of the family dynamics unnoticed over time.
I say this because you have written about the narcissistic rage and volatility of your DM and it’s not worth triggering that for you.
I would also not discuss your determination of strategy with any other family members. They may feel the same but more likely are enmeshed in the wide Narc family dynamic - any change or challenge to which feels like an emotional threat to then even subconsciously because the Narc keeps everyone on eggshells with their threat of rage, game playing, manipulation, smearing and ostracising.
You don’t need anyone else in the family to validate your experience so don’t seek it there - you will be disappointed.
You don’t need to JADE (justify, argue, defend, explain) to anyone else in the family what you are doing. They don’t need to know - get support outside the system.
IME you may loose connection initially with some members of the family - or at least the dynamic will change - but this is likely to be emotionally healthier as enmeshed family systems are too close, interdependent and controlled by fear, obligation and guilt (FOG) set up by the Narc.
Have you looked at each of the regular interactions you have with the individuals and them as a group and decided how you will reduce information and contact / change boundaries incrementally for each?
Maybe decide over a year to do this around day to day stuff and events (Mother’s Day, birthdays, holidays, Xmas etc). Scorched earth approach will only add drama and backfire on you IMHO.
Feel confident that you know what’s wrong and that you are getting out of it.