Mentally retarded made to sign on a release deed how to challenge it

My uncle mentally retarded has a share in joint property. My aunt made a release deed in which he was made to sign, in that release deed his share of property was transferred to my aunt without any notice to other siblings,the very next day my aunt executed a sale deed of his share took the money and went underground. We dont have any evidence of my uncle being mentally retarded before the date of signing the release deed. What should we do ?