Retrieving gift deed from my grandmother to my cousin

Hello. My grandmother inherited her properties from my grandfather. She had already distributed most of the small properties among her children. She did not distribute all the main properties. She wanted to wait till her end. My father is the second youngest child and the only living son of my grandmother. It turns out she had DONATED a main property to my cousin (her daughter's son) without anyone's knowledge. My uncle's (father's elder brother) sons are all alive and it was bound to be theirs. But she had written it without the knowledge of any of us. Is there any way to retrieve it. She is about 82 years old and is somewhat mentally unstable.