Issues With Adoption And Gift Deed

Hi everyone, your advice would really help us. My wife is an adopted child to her parents and she is the only child. While she was adopted their parents have not done any paper work or there was no proper adoption ceremony. My father-in-law has transferred his self earned property to my wife through a Gift Deed. But my father-in-law's brothers are making it a fuss saying that she is not the legal heir. My father-in-law is quite old now and wants to make this completely legal. Is there anyway that we can defend their claim. If my father-in-law passes away they might file a case regarding the same. Can he give a testimony in front of the judge to avoid this in the future?? Please advise. Thank you.