They can miss the point in spectacular fashion in most conversations. They get things arse-about-face generally.
Coupled with their 'it's all about me' mentality, and the fact that they are always right, there is absolutely no point whatsoever in trying to get anything across to them.
When I speak to my mother or sibling, I have to plan exactly what I am going to say, to the point of writing it down so that it is in words of one syllable, and suitable for a 2 year old to understand. Even then , my mother cannot (won't?) understand anything I am telling her, and is getting dramatically worse as she gets more elderly. She really is morphing into a two year old.
My sibling is even worse as not only am I misunderstood, but most things end up with a huge tantrum and unpleasantness, as my sibling is a histrionic N. I restrict my dealings with them as much as possible.
My late father was the worst of all. You literally could not talk to him. He was closed off, nasty and violent. He used to say to my mother...'you are insane', and actually he was right about that! But he was too!!
Joanna Ashmum writes well about the cognitive deficits of Ns. It's very tiring dealing with them. And that's not to mention the days one has to spend in psychotherapy to save your own sanity.
There is evidence from scans that their brains are actually physically different to those who are not afflicted with this PD. And I believe that it is also genetically predisposing, from the evidence I have from my own family history.
On a personal level I also believe that there has to be one family member who is not PDed, as otherwise the family would be in chaos...someone has to keep them vaguely on the right track. They have no original thought, and cannot initiate anything, so life is very difficult for them, in all sorts of ways. The problem is that the non-N is driven literally mad by the Ns!
It's a conundrum to which I do not have the answer!