How to correctly set a boundary is to set a boundary and then a consequence for crossing the boundary. Make sure to express why you’re setting the boundary.
I.e. “when you talk to me like this, it makes me feel this way. If you continue to talk to me like this, then we will need to take a break from communicating .” And then follow through. You can give a warning or two if you’d like but it’s important to apply the consequence and follow through.
This is very good advice imo, for when you are dealing 'normal' people.
The trouble is that when you're dealing with a narcissist or sociopath, they will sooner or later overstep your stated boundary or do something else that you will have to set a boundary for and then they'll sooner or later overstep that... and in the end (for your own sanity) you need to go 'no contact'...
Then, unfortunately, a big part of the fall out from doing that is that you have the rest of you family and/or mutual friends to deal with that the narcissist or sociopath has been 'working on' to get them on their side (they love to manipulate everyone and are usually very good at it, it's all an enjoyable game to them and it makes them feel good and powerful and superior to everyone else, and sociopaths have a superficial charm that 'normal' people fall for so it's an especially easy game for them to play)...
The 'worked on' family and/or mutual friends then 'go to work' on you with saying such things as "oh, they didn't mean to do it, everyone makes mistakes, give him/her another chance" and/or "you just took it the wrong way, that's all, nobody's perfect, they were just trying to help" (or whatever) and then they point out all of your defects/flaws as they see them and then 'lo and behold' you become the villain of the piece and the narcissist/sociopath is a hard done by paragon of virtue!...
This happens because I've found that 'normal' people have a blind spot for narcissists and sociopaths, they don't want to believe that they are living within our midst and if they themselves are affected by one it can take sometimes donkey's years (if at all) for the penny to drop.
I didn't have a sibling that was one, but I did have a long-term boyfriend who was and also a 'friend' who was.
With my boyfriend, I was able to cut him dead without too much of a problem (we had no mutual friends and I didn't work with him or live very near to him). However, he did try all manner of tactics to get me to communicate with him, which I had to be strong about and resist (and it was very hard because I was still 'in love' with what I had thought he was, and what I wanted him to be).
With the 'friend' I also went 'no contact', however she and I were part of an extended group of ex-school friends and they were never all going to believe what she was... So, I didn't have a 'show down' with her and/or set any boundaries, which I knew she'd only break. I just all of a sudden stopped all contact with her and with regard to the mutual friends in the group, I opted for the approach of
'don't complain and don't explain'...
When each of them asked me why I wasn't in contact with her, I just said that she had majorly upset me, so majorly that there was no way back, there was no point in discussing it with her, and as I hadn't discussed it with her, I wasn't going to discuss it with them as that wouldn't be fair -- and if they pressed me to explain, I stuck to my guns and wouldn't. (That way I didn't get embroiled in any discussing of the rights and wrongs of the situation as they saw it, and I didn't suffer any criticisms and/or character assassination, albeit maybe some of that went on behind my back, but I never heard about it, and I just took the risk.) From then on I saw my other friends in the group individually. Fortunately, none of them decided to cut me off, but I was of the mindset that if they did, so be it -- as perhaps some of them would eventually realise what she was and make contact with me again.
I'm not saying that I recommend what I did regarding my 'friend' as the way to go for everyone's sociopath/narcissist situation... it's just an option to consider. All I can say is that it saved me a lot of extra unnecessary emotional upset and trauma, and self preservation is key, especially as when at the hands of a narcissist/sociopath we will have already have suffered badly and more than enough.