How to prove that father was mentally fit at the time of making deed

My father has 2 daughter and 3 sons. I am the younger son. My father, gifted one of his property, our residential 2 storied house to me, because he don't likes the other siblings of mine. The deed is registered. He was mentally fit at the time when the deed was done. If my siblings challenge this deed under the ground that my father's was mentally ill at the time of execution of the deed, which is untrue, how can I prove that he was mentally fit at the time if executing the deed ?