How to claim the land property from the maternal side?

Hi Sir, My brother and sister in law both have expired. They have one son and daughter of age 16 and 14 years respectively. Their maternal grandmother (earlier) and grandfather (later) also expired. They have 5 daughters (no son) of which my sister in law is fourth one and the last girl is mentally disabled. They have approx. 9-12 acres of agricultural land. This land again part of it is inherited from the grandmother family and part is self purchased. I heard that the remaining three sisters of my sister in law have taken off all the land and distributed among themselves and are enjoying the property. They are residents of the same village. So i don't know whether the grandfather (in this case) had legally transferred the property to all the 3 or not. If transferred, still these children has a right to claim the property?? The whole maternal family of the children completely avoided them because of the reason they need to give the share. In this case children are minor and are being looked after by my parents. At present they are in a crucial stage of their education. I think they should not get disturbed by these things. So in this case what is the right time to proceed so that they can use their rights to get their share from them with minimal disputes. My parents are old and moreover depressed a lot by my brother's death. They are not that much energetic and active from last 3 years he passed away. So i should get into this. But i don't know how much time, money, pain it takes to proceed. Please suggest me.