Gift deed from Mother but father had acquired the property

Hello, I have bought a residential piece of land from a seller who had the property in his name through a gift deed registered in sub-registers office. The gift deed was executed by seller's mother on 16-11-2011 in favor of the seller , the property was self acquired by the seller's father.. On death of seller's father, his mother has gifted the property to the seller. Seller is their son by means of adoption. Seller's farther had 4 brother's. What hold does the children of these four brothers have on this property. The seller's adopted parents didn't have any children. The children of seller's father's brothers have put a claim now on the gift deed that the gift deed was executed by forgery and fraud. Its been more than 3 years after the giftf deed's execution that a claim has been put forward now. What are the chances that the gift deed could be set aside by the court. We had made sure that the seller's adopted father's death certificate is available and that the gift deed has been registered duly. Please advice