signing at death

My father had demitia and parkinsonsin, he had under went foot surgury and had to go to a nursing home for physical theorpy and because he could not walk for six weeks and my mother could not care for him. During this time my mother found out she was in stage 4 with lung and bone cancer. My sister was having papers signed by both my parents which signed a vehichle over to my sister , made changes with their bank accounts, insurance company, ect.
My mother died 81/2 weeks from being told of her cancer and my father 51/2 months after her.
My father was in almost perfect condition according th the doctor who performered his foot surgury. His cartilidge was almost gone in his left ankle and they put pins and rebuilt. He was in the beginning stages of demitie and the parkinson was very light. After the surgury my father was confused and seemed to have degressed with the demitia , putting him in stage 2/3.
My question, my parents attorney was present at each signing of papers, my sister was appointed trustee by my parents, but she managed to have her name on everything, giving her complete control to buy sell, ect. without the attorney. How do I prove this and can I have her removed as trustee.