My sister coerced my mother to sign over her trust to her. There is lots of foul play and strange twists. What is the best way to proceed?

My sister has isolated and separated my mother from all close family relations and friends including me, her son, my kids, her grandchildren, her niece whom my mother was a second "mom" to which broke her heart. My sister claims that all of us, including my mom, emotionally abused her and then told my mom that for retribution for said abuse should be giving her the whole trust. My sister used a therapist to convince my mom of this abuse which there has been none. We got a letter two years ago through this therapist supposedly written by my mother that if we did not apologize to my sister in writing and admit guilt that me and my wife and children would be kicked out of the family. We have never ever spoken to the therapist. None of us did as asked as we have no guilt in the matter. My sister has had a pampered life. Recently I got a note from my mom saying that she wanted to live with me and that she had signed the whole trust over to my sister. I have many more details...