Hello Hope for healing,

Welcome to the Parenting board!
The situation you are describing of your daughter getting worse with added responsibilities and stress of having a child is rather common... .However, it must be distressing for you to watch that and the possibility of separation and all that would entail for your little granddaughter (gd)... .
How are you coping with this, are you having trouble with the same behaviors from your daughter, or is it mostly isolated to her intense relationship with her husband?
We are here to support you in this and this is a great little community of parents and grandparents with lots of resources (workshops, articles and book recommendations) on how to deal with the issues. If/when you feel up to it, the best place to start is to go through the right hand panel with the
TOOLS and
LESSONS ---->
How is your communication and relationship with your son in law (SIL)? Do you think he would be open to joining this site? We have a board for spouses and significant others who are all working through their situations and if he is interested, it would be of great help to him to learn to deal with his wife's behaviors regardless of where he takes this (intervention or not).
As far as the question of intervention, it is a tricky thing, because every person is unique and so it is hard to predict what they will do. Plus, even if they recognize that they do have a problem, are they going to get motivated to want help and work hard on themselves or not?
Here is the dilemma with your daughter: you say that she has BPD and maybe even NPD traits. NPD unfortunately makes it even harder for a person to admit to themselves that they have a problem. And as long as they can blame others and get what they need, they will continue to do that. If she is faced with a difficult consequence of having to work on herself or lose her family, it may force her to do that. On the other hand, she may double down, dig in her heels and see your SIL as an enemy to be destroyed at all cost. It is a big risk for sure.
What I am hearing from you is that your SIL is at the end of himself and something needs to change... .
The bad news is that we cannot control or change others. The good news is that we can change ourselves and the way we act in the relationship. That will change the dynamic and the person w/BPD will be forced to adjust, and the relationship itself will change. If your SIL learns to protect himself and his daughter from the negative behaviors of his wife, he and your granddaughter will be healthier for it. There are no guarantees of positive outcomes, though for your daughter or for the marriage itself. Only your daughter has the power to decide she wants to change.