My sister has removed our mom from her home in order to prevent mom from witnessing changes made without her knowledge. What can I do?

We need to know more. Where is Mother now? Is she going to return to her home? What changes were made? Why?

What kind of changes? Cleaning, organizing, installing safety equipment? Sometimes, people who have dementia may not tolerate loud noises, strangers or commotion. Without knowing more, I'd wonder if she had good cause.