Sibling has POA without my knowledge. What can I do?

My mother is in the beginning stages of Dementia and it has come to my attention that my brother has POA. Obviously she had to have signed something and I assume she had no idea what she was signing. She has called me several times quite upset because she was told by my sister in law that she is no longer to open her mail. Furthermore, my brother recently removed all of my deceased father's legal paperwork form her safe and took them all home. Because of her confusion, one day she is ok with this and and the next she is extremely upset and calls me crying feeling that she is no longer allowed to do anything and make her own decisions. I feel that the could be something fishy going on here and since I do not share any POA with him, I have no way of knowing if they are doing something unethical with her and my dad's money. Any suggestions?