selecting different payee for disabled adult

my relative,who I have been payee for her life,she is now 24. A relative has manipulated her into going to social security and having her be payee. She is suppose to be seeing doctors,DVR,REHAB, and also a therapist and none of this is being filled.She is also with the DDD. Please tell me what should I do. I thought about going to the social security and opposing their decision to change this. The other payee has gone as far as to keep my relative from contacting me. The other payee happens to be her young sister.