What is the validity of a Will made by an infrim and sick person

I and my brother consisted a happy family with my Mother. In 2006 my mother faced CVA and Had Paralysis with Lack of Speech, Locomotion and others. My brother is a drunkard with 2 wives ( first one doesn't contact with him any more having a child of 19 this year). According to doctors ( Mainly CT Scan Report)My mother was incapable of thinking and as she lost her speech he took advantage and made a Will. That useless paper was made 13 days before the day of death of my mother . Can you tell me what are the success percentages of the case merits and demerits. Will be happy to get a reply.