Welcome back
Wind Wind 
I'm a recent returnee myself and your post spoke to me. I have one grandchild, my GS will be 7 next month and, until recently, he spent every Friday night at my house and I returned him late afternoon on Saturday.
This is breaking myheart. I want to be a safe place for them.
Boy, do I feel this. My DD is not always kind to my GS and I'd heard that "one positive person in a kid's life can make a difference," so really did my level best to be that person for my GS. That ended almost two weeks ago. I'm so sad about it and so worried for my GS.
You say you're scheduled to see them the week of the 20th. How did that plan come about? Will you see them with your daughter? Your ex SIL?
I was actually one of those grandkids (mom is BPD) that didn't see my grandma for many years, ending when I was around 8. BTW, I turned out fine and my mom never did brainwash me against her. My grandma was my favorite relative, so maybe she made up for the crappy way my BPD mom treated me.
This is all too familiar. I am your grandma right now. I'm so glad you had someone like her in your life. Unfortunately, I don't hold out any hope that my DD is not brainwashing my GS against me. I can't imagine what he's being told about why he's not coming on Fridays anymore.
Grandchildren being withheld from grandparents is a real heartbreak for everyone and I wish us all peace and hope as we move forward.
Thank you for sharing your stories; it does help to know I'm not alone. Still sucks, though.
~ OH