So what about divorce with children when a borderline is involved. This is obviously tricky, as the borderline hates you with a passion. There is a really high probability that she will try to alienate the children with respect to you. This can be extremely difficult to handle. Extremely. But one must keep their head.
The best approach that I have seen is the one used by my brother-in-law. It is simple in concept, but takes patience and discipline. However, it probably only works well when the borderline is not particularly clever. Anyway, here it is: always act calm and rational, always be in control of the situation, always be looking out for their best interests without advertising that you are doing so, always refrain from criticizing the ex (within limits), be the rock that the kids need. Probably, you will have custody at least every other weekend. This hopefully will be enough to show them what the normal world looks like.
So if, as with my BIL’s ex, she can’t control herself, causing her to do stupid things, they will see it. Hopefully, they will catch on that Mom is loco. Not only that, they will see the consequences of her decisions, and decide not to be that way. It may be a tough education, but it is one that they will never forget.
If the borderline is really clever, I am not sure what to do. Perhaps a commenter might have an idea.
Addendum by Deti:
And document. Document, document, document.
Document every interaction with the BPD. Every one. Summarize them. Make notes of what they said, direct quotes if possible. Make notes of what they did. Every outburst. Every statement. Every out of control thing they say or do. Every one. If they start losing their shit, whip out the phone and start video recording. If they object, say that it’s because you consider them a threat to you and the kids.