Property Case

my grand father did family aranagement ( agreement + affavidevit + registered will) and divided property among 3 sons ( elder - ground, middle - construct with own expenses 1st floor and younger 3rd floor.) after 6-7 yrs younger son had sold out his share (3rd floor) using same agreement. As grandfather was living with us, my father asked him to transfer 1st floor without knowledge of anyone to his name, so for this grandfather made sale Deed for ground floor & 1st floor and after that my father sold the ground floor and transferred the 1st floor to my name using sale deed. Now, as my father middle brother is still living on 1st floor from last 30 yrs and he is paying all the house tax + other bills too has filed the case against us to cancelled the sale deed as he is the owner of the 1st floor based on the family arrangement. So, what is the chances of his win, I know we did some kind of fraud and he didn\'t know about this till date.