Grandmother sold property of my grandfather

My father was in govt. Service and retired on 31.01.18. We are residing 300 km away from my ancestral house since 1983 as my father was in service. My grandfather owns a property in city in his name away from ancestral property. He died in 2001. We are hoping that after retirement of my father we will construct a house at our portion of land after partition of land and so visited the site with my father after a long time aprox 20 years. But we surprised by finding a three floor house constructed at our land. When we asked they told they have ownership of land since last 12 years. We return back to our village and ask for the story. My grandmother is alive and she told that my younger uncle pressurised her to sign and sold the property paper without my father's concern. My father is 4 brother and 2 sister. And nobody bother for the mishappening done by my younger uncle. I want to simply know Q. 1. If my grandfather is the sole owner of the property can my grandmother sold it without our concern if there was no any will, or documents on whose basis property is transferred to grand mother. Q.2. It happens almost 12-13 years the construction is there and in that case is there any section in law which gives me power to file case against the purchaser or the accused