How to face specific performance filed by my uncle on my mother

My Grand father announced 1 Acer agriculture land in 1976 at the time of marriage.In 1989 he and my uncle made registered gift deed to my mother.In 2018 my uncle filed specific performance suit stating that my mother sold her land to him on same day the gift deed was registered.He paid all amount.He took signatures fradulantly on stamp papers.My grand father and my grand mother paid crop money to my mother till 2013.Both passed away.Now my uncle took control of entire property and started litigation.My mother filed partition suit in 2016 and he filed this suit in 2018.He fradulanty got survey number on his name.