Can relative claim right over land when there is a registered will

Hi, My father was adopted one of father's uncle( Chacha ji) because he did not have any own child. My father was taking care of him till his death like his real father.He made a registered will in favor of my father in 1996 that his all the assets and property will be transfer on my father's name after his death.According to that will mutation of agriculture land was done on my father's name in 2007 but now after 9 years of mutation, my father's real brothers (two out of six) claiming that they also have the right on that property. They offered some money to Tehsildar and sent a summon to us. I would like to know the answer of few questions: 1.If my grand father made a registered will so how can they claim for their right? 2.What actions should we take against them? 3.In Indian law what is the importance of a registered will? 4.what would be the circumstances of this case. Please guide me. Looking for valuable suggestions.