You need to stop fighting with her. You are trying to reason with a loon.
OK, so you have put your point across. You never know, it may work... probably not, but you tried. Now carry on taking the high road and refuse to engage. Keep saying over and over, no-one wants to fight with you, but your behaviour is not acceptable. I've said what I need to say, and until you are able to be civil, there is nothing more that needs to be said.
Your dad will always be your dad, dead or alive, but she will never be a relative to you, unless she goes out of her way to integrate herself into your family unit. Sure she can make an effort to be a part of the family, but she needs ultimately to be accepted by you, and that's what scares her. She is threatened by you/your family, so wants to drive you all away.
She is clearly insecure, accusing your Dad of all sorts too.
The problem is that he is allowing/enabling her to behave like this, and worse with his own children. You ought to tell him that he wouldn't have stood for this behaviour from you as a child, tantrumming, abuse etc, so why is she allowed to do this to your family.
He needs to sit her down and tell her that this crap ends today. That he has a family, that that family is a dearly loved family, that needs him, needs his support and that he is free to give it. She needs to support him 100% on this, as he would on her if it were her family in need.
If she is unwilling to do this, then perhaps he misjudged her and made a mistake.
Then all he needs to do is to sit back and wait for her to step up to the plate.
If she doesn't, then it's over.
He'll find someone else, there are many, many single/unattached women.
Your family needs to band together and stop dancing to her tune. She needs to grow up or piss off.
Remain calm, she is not worth your blood pressure!