Fake signature of land owners in the Site plan of a building

My father and father's friend bought a land side by side to build their own houses. In 2010 my father's friend passed away. Three promoters reached us my father and father's friend's family and convinced them to give the land for building flat and two owners will get one floor each in the building. The agreement happened in 2012 via notary. The construction work started but after few years the three promoters had some disagreement and construction work stopped. One of the promoter is my father's friend's brother in law so he managed to provide one floor to his sister in 2017 which is almost 100 percent complete. My father got nothing till that time. My father reached one of the promoter on 2021 or 2022 and he said of he doesn't complete the work of one floor in 3 months he will give keys to the 1st floor which he took advance from a customer. He failed to complete the work and gave the keys to my father. After some months that promoter died and his son took the responsibility to complete the work. His son also failed to do so. My father acquired those flats which keys are given. Now my father's friend's brother in law along with her sister and other promoters pressuring us to sign a new agreement but denying to provide any compensation. Crime 1: failed to deliver the flats which was mentioned in the agreement for 12 years which is just a notary. Crime 2: the promoters has done fake signatures of the land owners in the site plan of the building. The sign of my father is fake. My father's friend who has died on 2010 has signature on a 2012 site plan and 2014 renewed site plan. Obviously his family has done that along with the promoter. What action should be taken to deal with this situation? Kindly help.