What if someone misuses the power of attorney?

I am from Tamil Nadu.20 years ago my dad bought a plot adjoining our house. The plot was originally a gov(poramboke) land that was allocated to a person A by the the Tahsildar since he had been possessing & using it for years. He sold the land to a person B with an unregistered sale deed. Person B sold it to person C with a registered Sale deed. My father bought it from person C with a registered Sale deed. Suddenly the grandchildren of person A claim ownership of the land by getting the Patta transferring the title of the land from their grand parent to parent though Legal hair certificate and they are trying to register power of attorney to a builder. How can we save our property?