Hello Rene,
It does sound heartbreaking to witness what's happening there. I also sense that despite this, you do have affection for your DIL, and like your son, probably are struggling trying to reconcile her fragmented or compartmentalized personality.
What happened with the cops, and s everybody safe right now? Was it suicide ideation, domestic violence or both?
3 months is such a tender age. I remember my Ex, then some months pregnant with our daughter, storming out of her mom's house on a Christmas Day, leaving me there with our then almost 2 year son. I shortly got a "yellng" text, "Bring home our baby!" I arrived to find her curled up on the bathroom floor, utterly despondent. I found what sounded almost like a suicide note on our computer. I was very close to calling 911. I didn't know what to do. These situations are scary.
What I can encourage you to do is have your son register here and post on the Improving Board. He can get "live" support. Sadly, this story isn't uncommon :'(
It's good, crucial even, to have a safety plan.
Safety FirstYou can pass this on. He should be able to access it without registering here, as well as these lessons, which are extremely helpful:
Lessons for Members who are in a RelationshipThey can help you and your son learn more about BPD, and Lesson 3 contains the communication tools, developed by leading experts in the disorder, which can help reduce conflict with a high conflict personality. Validation goes a long way (and this tool will help you when the baby starts growing up, too

Lastly, we've found that mentioning BPD as a diagnisis, usually triggers worse behavior. BPD is in art a shame-based disorder. Protect yourselves and your precious 3 month old GD as you can. Let me know if this is helpful. I know it's a huge amount of info, so ask whatever questions you have.
Turkish