How can a son get legally involved to help his Mom survive her days as she is getting where she will need constant care?

If an elderly parent has an adult child and this child did something that ended up in her having a lengthy mental evaluation, can this person still be capable of being "in charge?" There are two heirs, this daughter and a son, both in their 40's. Sister and mother have always disliked the son, and things changed drastically for son when father died. This sister, who was just released from this evaluation, will not talk about what she did. There is question of a fire being started by her. Son just wanted to care for mother, and was doing so. He did not want to have to fight this but he really has no choice because his sister said she "cannot handle anything.” She is depressed; she cannot get up in the mornings. She is trying to change mother's thoughts and has caused her to worry about everything now. Son said then I guess I will try to get P.O.A. to help mom (pay her bills etc.), and that is when sister went off in anger saying she would FIGHT HIM! He is a very good son, and had been attending to his mom's needs every day after & before work. Additionally, if this sister really started a fire in (that is what is believed happened to initiate her evaluation; she refuses to tell her brother what she did) it worries me that she could possibly hurt their mother. The son has been employed for 25 years, has very good credit, and no felonies. What can he do to help his mother? The daughter has a close relationship with the attorney that handled father’s estate, and helped with mom’s will and trust. They have done all this without even consulting the son. As an heir, doesn't he have a right to know what is going on? He was taking care of his mom, the sister moved out of state years ago because she blamed mom for her troubles in life. Just recently came back, brought attorney to moms house, and attorney told her the home was not fit to live in (not true), plus scared her about everything under the sun. She thinks she is broke, but is not. I find this attorney very unethical. His sister will not let him be involved through this attorney “friend” and now they hired another attorney for POA of finances/medical, then sister LEFT to go back to where she is living (some type of halfway house after being released from her LONG mental eval). They came and made mother write all sorts of checks, and she does not even know what for. This NEW attorney has not even contacted the mom, and she already has a bill from him for over 2 grand! We even emailed him to ask if he is in charge now so we will not make double payments on bills etc. for mom, but he will not answer us. We prepare food for her because she is a diabetic and is getting very frail. She aged drastically since the daughter came to town, it is shocking. How can an attorney legally listen and follow direction from a client's daughter who just got out of a very lengthy mental evaluation? They seem to have manipulated this elderly lady. Now they have just abandoned her! And they specifically stated on a document to this NEW attorney that "mother doesn't want son taking care of her". I asked her and she did not say that and DID NOT want to hire another attorney! Her son IS taking care of her, before and after work...but this cannot continue as his mom is getting worse.. How can the son, who is caring for mom daily, get a legal standing on all this?