Shes been picking on my children from my first marriage. She said she is an outsider, outcast and my children hate her. All of it is not true. But no way i can convince her. She just insist. Repeatedly fights over it
Yes, that is a tough situation. There is a limit to what you can do, but let me suggest you aim for the middle ground.
She's insisting on a bunch of stuff that isn't true, that your children hate her, for instance.
You can't agree with that--it isn't true.
But she is really feeling horrible, hurt, and rejected because of it, and her feelings are real.
So if you tell her it isn't true, (to her), it is like telling her that her feelings are wrong, what we around here call invalidating her feelings. And that is unpleasant for anybody, and if she has BPD, probably really sets her off and makes things worse.
Your easiest middle option is to just refuse to discuss it with her, as it just gets her riled up and makes things crazier.
Are you on a vacation with her now, or do you have one scheduled soon this summer that you are worried about?