It's a sad story if true, but I think there has to be a degree of personal responsibility.
The WTS can't really shun anyone - who cares if someone you don't know doesn't want to talk to you? The only people who could really shun your mothers daughters (which I find an odd way to describe your sisters?) are their friends and family.
The person that made the decision to shun them was your mother. She took that decision, same as deciding to bring her children up in that religion in the first place without spending enough time researching it. But all those decisions are on her. Sounds like she maybe regretted it.
But now you are doing the classic trick of giving the WTS all the power and excusing what your mother did.
The fact that you see all these things that happened under the guise of religion, but are so adamant that it's all true is frankly mind-blowing. You can't see that maybe the entire thing is all an invention to control and manipulate people?
Your mother fell for it and you're falling for a different version of it by a competing sect.