Does gpa to gpa legal?

My grandfather had got a GPA from a seller for a property which government had granted and made a statement not to sell it for 15 years, due to this my grandfather made a GPA to my mother. My grandfather and the seller died before the actual registration. One of my mom brother stole the GPA which was made by seller to my grandfather and lodges a case stating that this property belongs to him by making another GPA . Here seller family which includes seller (before death), and his family members (wife ,son) signed the GPA agreement which was made later and made a statement in court that they the property belongs to my mother's brother. We were paid only 2percent of actual worth of property for the GPA that my grand father made to my mother by a politician who is buying this property. Consulted few lawyer. They suggest this case is very challenging and they won't risk it as it involves politician, gundas. Is there any possibility of getting it back by loding a case?is there any possibility of winning the case and probability of percentage in winning this case? Expecting a favorable reply.