Hi Lad's Lilla,
Thank you for writing in . You are starting to read "Stop Walking on Eggshells" which is great. Here is a link about grandparents with BPD adult kids on this forum:
https://bpdfamily.com/message_board/index.php?topic=145769.0 . It is a gut punch when the adult BPD child holds the grandkids hostage. Please continue to put your safety and well being first , especially since there is violence. There is really no real way to figure out what triggers the BPD adult. It would be good for you to put the focus back onto your self and your healing.
Please keep writing back to us as you are able.