Illegally encroached

My grandfather had 6.5cents of land and his brother has 3.5cents of land. My grandfather's brother has illegaly encroached 6.5cents of land from my grandfather and built a house in 10cents(around 40 years ago). My grandfather is illiterate. My grandfather had asked his brother whether he has taken any piece of his land, for that my grandfather's brother has denied. After the death of my grandfather in 2003, my father came to know that 6.5cents of land is in his name. Now my father has the document, patta and been paying tax from 2003. But my grandfather's brother is claiming ownership. No one is living in the house for past 15+ years. Now my father want to build a home in that property. Please help how to proceed legally.