Your father was legally adopted by his grand father
2) giving and taking ceremony was performed and deed of adoption executed
3) after adoption your father would inherit his adopted father proeprty
4)According to Hindu Adoptions and Maintenance Act, 1956, after adoption, the adopted son/daughter lose all the rights of a son/daughter in their biological family, including the right to claim any share in the estate of the biological father or relations, or any stake in the coparcenary property.