Interesting thought on this:
My brother still attends meetings but that is it. He goes with his family. I haven't done anything for 25+ years: inactive fence-sitter and proud of it!!! My sister & her husband are full-bore, do-it-all Dubs.
So, my Rutherford-ite, In Yer Face Dub Dad wants to unilaterally make up with me, while cursing my brother all the way! (Like you, I don't understand the personal rift, there.)
I led the way in boycotting Dad: read that 'reverse shunning'. My brother followed suit after Dad redoubled the pressure on him. Soon after that my sister (seemingly doing everything to please him) couldn't take his interference with her family life and quit talking to him. That leaves my brother-in-law as the only one to communicate with Dad.
Anyway, we did it to him, in self defense. It was actually suggested by the 'Olders' that we simply avoid interaction with him. He is a known trouble-maker, but an old 'pillar of the community' JW-wise. He started the local congregations, 50 years ago.
I feel that he should have been DF'd years ago, but when it's really needed, THEY MISS THAT ONE BY A MILE. (In an unusual tact, the 'Olders' all backed off, apparently roped us all off and declared it 'a family matter'. The family is a mixture of status, good to indifferent to "1000's of miles away".)
What I am going to suggest (and it is only a suggestion) is to 'do it by the numbers'. You apparently talk to both your brother and mother. You have solace and peace with the rest of your family. In fact, you are probably a great comfort to them. You should continue that.
You are able to 'walk into all camps', based on a technicality. You never committed to something that the other three did.
Your sister is the one that is doing the harsh thing. You are extending the 'olive branch' and supporting her. This actually encourages her. I'm not sure that continuing to interface with her is in the best interest of all concerned.
What we have here is that she is punishing the relatives by NOT EXERCISING THE OPTION to speak to them for family business.
Ever notice how 'legalistic' all this stuff is? The WTS is first and foremost just that: 'legalistic'. It appears more and more that that is more important than scriptural concerns.
Anyway, your sister is not entirely technically correct according to WTS. It is my understanding, (correct me if I am wrong) that SHE HAS THE OPTION of speaking to the mother (or even the brother). She is not allowed to discuss spiritual matters. (Someone noted recently an 'Older' who spent a vacation with all his DF'd children and declared it family business.)
My point is that I doubt that you will suffer as much by backing off from your sister, as your mother & brother are suffering. Your sister, voluntarily doing an unloving thing, needs some attention (or lack of attention) directed her way. You can even point out that she is not doing the loving thing by electing the harsher course of action. What she is doing is not strictly required by WTS, though some local Olders may push it.
And throw that nonsense about 'we are doing this harsh action to them for their own good' out the window. All that amounts to is an excuse for inflicting some self-gratifying punishment to someone.
And, in anticipation of hearing the moral condemnation of 'you will be no better than they are', I say HAH!!! So what? I'm in a bad situation and need to deal with it. Such platitudes only make you the victim of dangerous people by holding you in a state of inaction.
Mustang
Peace and best wishes