Simply getting back together like nothing has happened is confusing.
I know how you feel. This is my family's M.O., too. At least two generations on both sides have BPD, NPD, or both. There's a lot of pretending nothing happened.
I looked back at a couple of your other posts and see that there's a cyclical pattern--both in their behavior and yours. At times I have felt like I speak an emotional language my family can't understand. I got stuck for years trying to communicate with them to fix "the issues," to get them to see and heal my pain, to work things out like healthy people are supposed to. I've realized that my parents just aren't capable of that. One aspect of NPD especially but also of BPD is a need to protect the image of the self. My parents are not psychologically able to believe the facts I present when I have confronted them--they say I am making things up or that they could never do such a thing. They do not have the empathetic skills necessary to understand that their behavior caused me great pain. They don't do conflict resolution--they do conflict denial.
Part of my healing and my growth has been accepting that the kind of closure and resolution I used to want is probably not ever going to be possible. The new closure and resolution I have found is in accepting that they are who they are, and in letting go of my desire to make them see and understand me. Do you think it might be possible to have a relationship with your family that doesn't excuse or deny their behaviors, but accepts that they are not likely to change?
I don't think your brother is having a "temporary retreat toward partial sanity." Based on what you say your counselor has told you, he has a personality disorder. That is not something that comes and goes--it is part of who he is. There will be times when he functions somewhat well, and times when he doesn't. My advice is that you don't have the ability to improve your family all on your own--not in the sense of working out the deep underlying problems--and that your family don't seem to be ready or able to do that work with you. What you can do is accept that it might need to be a surface-level family. It might need to be short, casual, once-in-a-while get togethers. It might mean they don't apologize or call you up to be close to you. What would that be like? What would that mean to you?
The last thing to touch on is your very valid question about how to speak to your brother's children. You're right that it wouldn't be helpful to tell them their father is crazy or to blame others. Some lovingly truthful things you can say might include: "I am happy every time I see you;" "It looks like you are doing a great job growing up;" "Anytime your dad and mom say it's okay, you're welcome to call or visit;" etc. You can love them in ways that build their self-esteem, respect their parents, and keep them out of your conflict with your brother. I don't know their ages, but it's pretty likely that they're not half as hurt as you imagine they might be about not seeing you when you said they would. They probably aren't upset with you.
When is your get-together planned?
