Land problems

My father and his brother inherited ancestral land and all documents have been registered in the name of the two brothers. My father lives in the UK and visits India approximately once a year. We have recently found out that his brother tried to sell the land and took money from an agent for some of the land in 2011. As he has not transferred the land or paid the agent, he was arrested in June 2016. The agent is now demanding that the land is signed over to him and threatening that he will pay money in the courts and get the land transferred to him. This is ancestral land and we do not wish to sell it and nor do we want to legally separate and give my uncle his share of land to transfer to a random person. All land documents have the name of my father (older brother) and my uncle, so when the agent gave my uncle money, he would have known that there are two brothers owning the land. Please advise what action we may take to stop this sale and protect our ancestral land. We are not aware of Indian laws and do not wish to lose our family land. Any help/advice much appreciated.