Father in law got land from ancestors now his brother asking share

My father in law have 25 bigha of land which he has acquired from his ancestor so the registration of that land is of ancestor origin but now he want to sale that land but his own brother is asking for share from that land so what he is should do. but the name of owner of that property is my fathers in law. His own father he had 50 bigha of land out of which my father in law got 25 bigha i.e 50 bigha divided between 2 brothers but now out of 25 bigha his own brother is asking for 6 more bigha from my father in law & my father in law is looking for sale of total land but his brother is not allowing him to do so?