We responded compassionately, but firmly about the boundaries set
This is amazing. It seems rare that siblings who have been pitted against each other could come together and find a healthy way to unite. Really impressive.
Eventually my mother has ceased these lashings out, but rather than replacing the lashings out with calm, loving or even neutral contact she has largely ended all contact.
I don't think pwBPD do what they do because they're choosing between a suite of, say, 10 healthy behaviors and 10 bad behaviors and opt for the bad ones. I think of BPD behaviors like "safety behaviors" that are persistent and entrenched and there's not a lot of choice. Learning to respond in a healthy way would require a lot of work to dissemble how your mother thinks about herself and whether there are alternative, safe ways to go through life. It's kind of remarkable that she responds at all, although like you said it' not the optimal response.
I asserted compassionate boundaries with my father and it's sort of how you describe. The optimal response is not there, it's more the absence of the negative thing. I can see him fighting hard to not lash out, which often ends up with him becoming angry and stomping off into a silent treatment. Since one of my boundaries is to not chase him or appease him, more often than not a visit ends up with me saying "see you later!" from another room. I guess for now the absence of yelling is about all he can manage and maybe that's the best he can do period.
My father at first seemed to try to support us, but we realized with time that his priority is upholding the status quo or “keeping the peace” with mom. It was disappointing to realize his faults as I believe I forced myself to believe that he was the emotionally healthy parent in order to survive as a teen. He has essentially joined my mother’s pattern and made it explicit that we must contact them both together and never separately.
I think of the non-BPD parent as the "normal range" parent who is in a dysfunctional relationship, and because of that, is often surviving on a day-to-day basis. It sounds like she is largely adopting a passive-aggressive approach and has put him in a loyalty bind.
I feel a tinge of sadness over how it’s unlikely that I can have regular, reciprocated, boundaried contact with my mother.
For me it's been a full-out grieving process, offset by counter feelings of relief and some surprise that I can assert boundaries with both parents and a sibling. It's not necessarily easy but like you said, the quiet and the absence of being randomly attacked and yelled at is priceless.
While I know that unboundaried frequent contact with my mom is what has led to so much trauma (and is clearly the lesser of two evils) it is yet challenging to accept the new normal.
Amen.