Clarification on purcahse of land

My father purchased land in mysore along with relatives been acquired from kiadb without our notice, later our relatives got Katha from grama panchayat, when we were struggling to get, our own relative blackmailed us in the name of KIADB along with few gundas and took GPA and signature from me and my father on few stamp paper, but my sister didnt sign on that reason she was not in, over that when we asked money he said if you speak anywhere about this first you loose land apart from that you will loose your life, thinking this were kept quiet, we have few audio clips on what he was talking about KIADB transaction, now he is constructing his house, is their any possibilities to cancel GPA also is their any possibilities to save our land or getting justice.