Authenticity of Patta Land

My grandfather had a land of 110 sq yards however didn't have any documents related to it. After my grandfather's death, my grandmother had partitioned this land into 5 sons with each son getting 22sq yards each. Everything was done on stamp papers without witness. My father purchased has purchased one of his brother's land with agreement made on a stamp paper. Although there were no outside witness involved, the seller, his wife and his sons has signed on the papers. My father had built a house on this land and received a patta from Govt in 2003. My father sold this house bearing patta on his name to a third-party due to financial reasons. Now the sons of his brother(now deceased) claim they never sold the land and are claiming half the share. They had also filed a case claiming we have forged the sale made on stamp paper to our advantage. Please help understand if it is better to proceed legally as my father is the legal owner of the house since the patta was provided by the govt. Also can we use the stamp paper without outside witness as evidence of purchasing the land from my father's brother?