Division of Ancestral Property

Dear Sir, the case relates to the property of my father's grandfather. My grandfather had 9 children, of whom only 4 had married (including my grandfather). Rest died before making any family. Because of being away on job, my grandfather (and his sons i.e. my father and uncles) had always neglected that property. My grandfather and grandmother, both are no more alive. Now that the other legal heirs of that property are trying to sell it, they are pressurizing my father and uncles to give up their stake for a nominal amount of Rs 3 Lakh, whereas the market value of the property is around Rs. 2 Crore (actually my grandfather's share should be 1/4th of the land value, i.e. 50 lakhs. As per the govt land records sent by them, my grandmother's share is being shown as 1.5% when it should have been 25%. We just don't know who changed the land records and how. Now my query is, how can one try to amend the faulty land records. And that too in the absence of any documents, since the property is ancestral? What can we do to stop them from executing any sale deed without our consent?