Hello BrokenMomof3, welcome to the boards

It sounds beyond stressful to find yourself in the middle of your two daughters, one of whom seems to have some delusions about the other. Delusions and paranoia can be difficult to work with -- as "normal" people, we believe that if we can just prove that the delusion isn't true, that the person will be content. Like you're experiencing, that isn't the case.
Some members here have loved ones with BPD which tends towards paranoia/paranoid delusions. Many of the delusions seemed to fly in the face of reason, facts, evidence, and logic. Fortunately, many of those members were able to learn different ways to interact with their love ones' delusions, and have had some success in "not fanning the flames". So, when you ask "is it BPD or OCD", it's possible that this is a manifestation of BPD (the paranoid side), though I'm not an official mental health expert, so it's not a diagnosis.
A great place to start when you have a loved one coping with delusions could be Dr. Xavier Amador's work -- he published a book called
I am Not Sick I Don't Need Help! which might be a good fit for your situation. We also have an article about
Anosognosia and Getting a "Borderline" into Therapy which might resonate with you -- Dr. Amador's work shows up there, too, and there's even a brief video.
Your care for your D and your desire for her to be balanced, grounded, and well, really shows up in your post. While it might be a long road, it is very possible to learn some new and different ways of interacting, that can be just that for your family -- balanced, grounded, and well.
Keep us posted on your thoughts about the articles and video, and how you're doing in general;
kells76