Son in law cheated grandfather and transferred property on his name

My grandfather had purchased a property at Bangalore in the year 1979 by sharing the property(30*40 site) with his son in law i.e half site purchased by my grandfather, since he trusted him lot, he use to give money every year to pay land taxes (Katha), he was unaware of the proper rules of the land, the electricity and water bill was coming in the name of his son in law, and in 2003 my grandfather expired , in 2011 they forced us to sign on few documents, and now the property has been transferred on his name ie son in laws name, before doing it they changed it as gift deed in the name of my aunty i.e elder daughter of my grandfather. I want to fight for this property, but I hold only sales deed which was purchased on 1979. I don’t know if I log a case will I succeed in it or not. My grandfather have 5 children(2girls , 3 boys). And I’m one of his grand daughter Questions: Where should I get EC? What all documents needed to log case on them? Since we all signed on the paper?