How to claim property when living more than 40 years and paying tax

A case filed by my grand father's sister's son on My grand father's property, which is now belonging to my father and his brother. My grand father(died) has 2 sons, my father and his brother, (his brother died). (grand mother also died) My father is paying tax for that property for about more than 40 years, and also looking after that property from my grand father's time. Grand father had 3 sisters and 2 brothers, all of them died, now their sons filed case that they need a share of my father's property.. Actually there is no separate document with us that this property belongs to my father and his brother.. Only a document for the whole land which belongs to everyone in their family, (everyone includes grand father's brother's 4 sons and grand father's sons(my father and father's brother ). When we tried to build a house in this land, they filed case and asked for share. Building work stopped, the case was filed in 2013. Still no progress. We have to sell that land, what will we do?