Validity of General Power of Attorney after death of principal & agent

My grand father was allotted a land in a society in delhi in 1985. He constructed ground floor on it . He entered into a Genaral power of attorney with his brother for a consideration of Rs. 1 lack and he gave his brother the right to construct 1st floor over ground floor to his brother. His brother constructed 1 st floor out of his own funds. My Grand father's brother did not registered the GPA in his name. My grand father retained the roof right to himself only. As on date both the parties to GPA are not alive. but the 1 st floor is still in the possession of my grand father's brother family. I want to know today about the validity of GPA entered between the parties that are both dead. My father had mutated the legal title of the whole land in his name in 2016. How can we vacant the 1 st floor from illegal possession of my grand father's brother's family