Oh mulranno, I'm so sorry that you're in so much pain. My mother died of ovarian cancer 6 years ago; it is a terrible, terrible disease, and your post of 22.05 has brought me to tears.
After my mother's death, there was an appalling row in my family relating to the speed with which my father became involved with someone else; it was complex (I can't be more specific on here) and involved several members of the family taking different sides.
I know for an absolute fact that I did not behave in a way that does me any credit, and I deeply regret it (it was more a sin of omission than commission, but nevertheless, I wish I had acted differetnly.) The fallout is still being felt - and relationships have not entirely recovered, though I believe they will in time. I feel, in truth, that I let the memory of my mother down in some undefined but important way.
I think that what most people are trying to say is certainly not that you have to take it on the chin and allow this woman to behave as abominably as she wishes, but rather that she is almost certainly not going to respect any request to back off and will probably force a confrontation - one which, given her previous form, could escalate to no-going-back levels. It's possible that you might behave badly, and she most certainly will.
What do the rest of your family think of her? Have you discussed her with your brother, ever?
I'm very, very sorry for your loss, and I do hope that you manage to scrape some small pleasure from this Christmas. I hope that your children bring you comfort - it is very hard being a grieving child, and a mother at the same time.