Should I try to remove my sister as co-POA and co-guardian of our mother who has severe dementia? If so, how?

My sister has moved thousands of dollars from the family trust account to her private account and would not return the money until I threatened to file charges against her. She also got a copy of Mom's last will with bequeaths and removed those items from Mom's house that she did not agree with the person to whom they were bequeathed. All of this AFTER Mom was diagnosed with severe dementia and the family trust and her last will cannot be changed.