From one grandmother to another, I welcome you here, Cleverclogs.
I can see how difficult this must be for you... .in between a granddaughter you are wanting to support while having to deal with your daughter's negativity in the situation. It would be so much easier if they didn't live with you... .but they do... .so it is in-your-face all the time. Not easy, for sure!
Have either you or your daughter gone for counselling yourselves? All of us here know how difficult it is to live with someone who suffers from BPD. As well as participating in this wonderful support group, many (including me) have sought out professional guidance to help in navigating through the journey.
It is good to read that your granddaughter has been receiving help and she will come home knowing of the tools she needs to use to keep her on-track but it will be so much easier for her if she has the support of those around her.
Your work is cut out for you, Cleverclogs. I hope you keep sharing. I hope you take the time to do your homework along the way... .so much info on this website

|--->and links to more. I hope you also get to feel the support that is offered from the others here. While all of our stories differ, there are still common threads... .something in a post that will resonate with someone else and a connection made.
Once again, I welcome you, Cleverclogs.
Huat