I just see it as they see the universe revolves around them, they think everyone is below them and should bow down to them basically and they just love themselves whenever anyone compromises this love or the way they view the world they can become unpleasant and even dangerous. This is a very skeptical view, I’d have a more open mind if it wasn’t for my psychiatrist in hospital who was textbook narcissist, he was dangerous, this isn’t paranoia talking as my parents knew it too he practically destroyed me and used my delusions to call social services on my mum, what his intentions were I do not know whether it was to ruin the relationship I have with her or to destroy her there was intention behind it not just protocol, I was mistreated and whenever I was with him expressing my distress he would put this smug smile on his face like he enjoyed it and my psychotic break in hospital which was my downfall he let me suffer for days before finally putting me on meds, probably to get at my mum because he made her beg for me to be put on meds. I’m thinking all this was due to a complaint she put in at the beginning.
I’m not sure whether the man you’re dealing with is a narcissist as some people can think like that without being the genuine article all I know is if he’s a narcissist you’ll find out, what I’d say is warn your sister to just be wary that he could be dangerous because they can be manipulative in sly ways. I’m sure your sister would feel it before she saw it for real, I did with my doctor, as our animal instincts would make alarm bells, I’m hoping the same is for everyone anyway as my brain isn’t very reliable :/.
I suggest having a serious conversation or writing a letter to your sister getting her perspective and expressing yours, you may have already but if you do so again maybe she’ll know you’re being serious. Take care J, I can tell how much this is effecting you, keep up your techniques and keep communicating!