Can my brother void a Trust our parents signed?

My parents took me to their probate lawyer to sign papers that if something happened to them I would get their home. My dad passed away last year my mom has dementia and now my brother has power of attorney. Can my brother void that Trust they signed? He is trying to get everything. He moved her into assisted living, my aunt and I don't know where she is. My brother won't tell us. I took care of them for years. Now my brother comes in for the last 4 months and is trying to get everything.