Please help my husband and navigate this situation. We love our son and grandchildren so much and are just devastated.
Hello and welcome to the family! This sounds like a devastating situation and it's a common topic on these boards. My heart breaks for you- especially due to the potentially fatal illness.
There is only one real path forward in these situations- making peace with the daughter in law. And trust me, I heard you...she's impossible. My best friend's little sister is exactly the same and she's written off literally everyone in her life as well (including parents, uncles, siblings, etc.). They haven't seen their grandkids in 7 years, and they'll sometimes ask me to reach out for updates.
It's heartbreaking, for sure, but you have to be the adult in the room because your DIL is incapable. Leaning on your son to fix things is also a mistake because that will just add pressure to an already impossible scenario...he's powerless to make things better and if he tries, then things will get ugly for him as well.
Here's a few things worth trying:
1) forgive the DIL for everything, no matter how ridiculous, because the relationship with the grandkids is more important. She's the way she is because of mental illness and you can't change that, but you can show her compassion and give her a little more grace than she deserves.
2) try reaching out with a peace offering, even if it means accepting unfair blame. When the BPD's in my life push me away or fly off the handle, I'll say something like, "I'm sorry for what happened and I don't want to argue anymore because I care about you. Can we move past this?"
I know what you're thinking- that's a terrible move with a BPD control freak. But there's one goal here- to end the conflict. If she says, "Well, you did this and that...." Okay, apologize for it and show empathy. Eliminate the need for that to be an argument or a line in the sand.
3) don't ask your son to fight this battle, but do offer him extensive support and help in any way, shape or form. What can we do to help? Are you eating enough? Could we grab some groceries for you? How can we support you? This indirectly makes it harder for the DIL to see you as the enemy.
I hope that helps as a place to start. Please ask away with questions so the family here can help!