Illegal sale of Ancestral property

Hi, We are having ancestral property which was sold by my younger sister and her husband without my consent, we had raised few notices published on newspapers. After those notices the people who illegally procured the property from forged documents/signatures contacted me and agreed on providing my share if I give them NOC/POA. Till now its not happened and it seems they are looking to sale the disputed property illegally again. I stay in Delhi from last 15 years and somehow I have no faith in the people i.e. neither buyer or the middleman whom I taking support on this. Property illegally sold in 2012 and last notice was published in July 2014, from there after the buyer offering my share to us but all baseless as now they are not taking calls too. We need advice how we take legal action against them and what is the deadline for challenging forge registry above all what will the overall cost involve in the case.