You'd probably have to show how it affects the kids or your/her ability to care for them... .
That's been my experience, too. Demonstrate how these behaviors have impacted the kids. Then get an expert witness to testify what the effects of BPD are on the children.
I learned how important it is to have a third-party mental health professional testify -- in your case it might be an expert witness. In my case, that person was a PC, and her observations of N/BPDx went far enough to get me sole custody.