Welcome, Bubbles404 -- you've found a good group of folks who "get it". Right off the bat, I'm nodding my head at this:
My brother is pretty high functioning, at least it appears that way.
Yes. Part of BPD is that for many, the traits can intensify as the closeness of the relationship increases. So, to work colleagues, teammates, distant relations, etc, the person can present as "with it", charismatic, insightful, magnetic, artistic, warm, loving, "together"... just "an AMAZING human being". It's in the person's closer relationships (children, parents, romantic,
siblings) that more of the disordered traits show up.
he experienced help in grad school from DBT counseling
I'm curious, how does he talk about this? I.e., does he seem to have insight into his own behaviors? Any moments of clarity, and/or acceptance of a diagnosis (if there's been one)? Or is there some sort of dismissal, like "Yeah of course DBT is the best for people who need it, I only took it to see what it was like for clients, I didn't really need it like they do"...
His response was “what do you want me to say? ‘Don’t leave’?”.
After I said, “I just want to know we’re okay”, he said, “we’re okay” and walked to the guest house and I have yet to hear anything from him.
His response was, “ I understand what you are saying. For me… I’ve never felt so alone or abandoned. I will work through it, I always do. I love you too. You are not going to loose anyone.”.
As weird as this may sound, I think I'm hearing more manageable, acting-in type behaviors, versus the overt lashing out/blaming/demonizing type, so that gives me some hope that there are some tools and approaches that can work in your relationship, as it transitions from coworkers back to siblings only.
Is that your experience, that he tends more towards the "I'm an awful person, I'm really down, just leave me alone for a while" type stuff, versus "you're the problem, I'm going to badmouth you and blame you to everyone, you're the devil" side?
He sounds extremely emotionally sensitive, yet also with at least some awareness:
I will work through it, I always do. I love you too. You are not going to loose anyone.
so I think there is hope here, long term.
Any advice, thoughts, encouragement would be great. I’m feeling a ton of guilt that the company could very well go under.
Fear, obligation, and guilt can be hallmarks of dysfunctional family systems. We have our "roles" that we must stay in, for the family unit to function and "present as functional", yet the price of staying in those rigid relationships is an overwhelming sense of guilt, and the reason we stay in the roles is the combination of fear and obligation. We may believe that if we change, "everything will fall apart". We may have beliefs that "they can't do it without me", or "if I don't do it, it won't get done". Let's just say I'm speaking from personal experience here
I wonder what you'd find if you examined that belief some more -- the belief that if/when you take the new job, the company will go under. As you dig a little, do you find any beliefs/thoughts about your brother and his capabilities, about "what good family members do", about anything else?
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I want to give you kudos for your commitment to taking the new job, too! You recognized a good move for YOU, and you're taking care of yourself. This is another thing that we from dysfunctional family systems can feel guilty about -- taking care of ourselves. Yet, how can we take care of anyone else, if we can't, won't, or don't take care of the person closest to ourselves first -- namely, US?
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Bubbles404, let me know if any of this is off target; I know we "just met"

. Looking forward to hearing how things have been going for you...
kells76