I am sorry for the loss of your father. I am glad for you that you could be there for him at that time. That's what matters the most.
Both my parents are deceased. They remained married. My father passed several years before my BPD mother. I think in summary- this kind of loss is difficult for even the most stable families. I think in families with dysfunction, it seems to bring this out.
Also, dysfunction affects every member of the family, even if some of them don't have a disorder themselves, because families behave as a system. Both you and your brother are reacting to this situation with the "relationship tools" you each have.
At the time of my father's passing, BPD mother was angry at me. She enlisted other family members to "her side". I have no idea what she may have said to them about me. I felt ostracized. I didn't initiate no contact, but they didn't contact me.
Like you, I was grateful for the support of friends and my father's side of the family. It seemed as if my family was split in two. I heard nothing from my mother's side.
This time was chaotic. PwBPD have disordered thinking and behavior. That your mother shows up as the kind, caring, grieving family member is her way of protecting her self image.
I did retreat from this for my own self care. If this is what you need to do, then do it. Take some time for yourself, keep your boundaries. This is a time of grief and loss for you. You don't need to be emotionally caretaking full grown adults (children do need your care- they depend on you and it's a loss for them too). You may want to consider therapy at this time for your own emotional support. I did it and it helped.
With your brother, I don't think you need to go full NC unless you choose to. He may not have good boundaries. On your part, you can still have them. You've asked him to not discuss your mother, or sister. Know that if he has poor boundaries with you, he also does with them and that anything you say to him is going to be reported back to them. You've been NC with your mother and sister and can keep this up.
You can control the discussion with him. If you have already stated you don't want to hear about your mother/sister and he starts- you can gently remind him "I can't listen to this at the moment, can we change the subject" and if he persists, politely get off the phone "I love you brother and care about you but I can't listen to this. I will call you later" and get off the phone. Essentially- you are teaching him this boundary while also letting him know you care about him and still want to talk to him. You also can control when you are available to speak, or text. "I can't talk right now but I will call you tomorrow" and decrease the frequency of contact to what you wish to have, for now. It may change later but you can prioritize your own self care.
Losing a father is huge, emotionally. Even the strongest person needs time to grieve and self care. Do this for yourself. Grief takes it's own course. Give yourself the time you need. Take care.


