Issue of my grandfather property

Hello sir, My mother was adopted by her father's uncle, not legally. Her parents are also alive. Her education, and marriage everything was taken care by her adopted parents, they don't have any children. My grandfather has joint land property with his brothers, they shared it that"each one gets 17 cents of land". My grand father built a house in this land. After she got married my grandfather registered that property on her name. But after few years son of my grandfather's brothers showed that my grandfather's property belongs to him with the proof of registered documents. Then my grandfather cancelled those unknown documents at register office. After they file a case that "how they can cancel this registration". They faced so many problems because of them. They hate our grandparents but they want their property. My grandparents liked to give that property to my mother whole heartedly. My grandfather is now in his 80's. Now can you please suggest me how we can proceed this case legally and how we can win this case. And I want to truthfully telling you that " their documents have my grandfather's not forged signatures and all." We don't know how they did that. My grandfather saying he didn't do anything. Please help us to make my grandparents happy.