Land Registered in My grandfather name but fathers cousins claims

My age is 30+ now,we have multiple joint properties in WestBengal,Malda area.Every property is in my grandfather's name.My grandfather had 4 sons,all of them has expired as of now, my father being younger of them. My father passed away in 2001.Now we lived in Kolkata area,so didnot have enough chances to visit and at that time i was 13+ years old(Minor),my fathers cousins(Grandfather's Brother's son) is claiming that they have bought 4 to 5 lands in the year 2002 and approaching us for land registry.We didnot have any idea regarding the same,and if my own uncle(one who lived in villages near property area) have taken any money from them,they claimed they have given around 5 lakhs for 4 lands,which stands at 1.5 crores in current valuation.I want to claim my possession alongwith other heirs of my grandfather(with my cousins brother),iam requesting my uncles who claim to bought the same,to give me 50% of current market value of the property.Now they are denying,how can i claim.