Sharing property

Hello sir, we are 3 sons and two sisters to my parents. 40 years back we all got married. Before all my siblings marriage,my mother taken care one of my relative,because of taken care my relative gave some crop land to my mother while my relative passed way(there are no kids for here). After all my siblings got married we got separated and during separation all properties equally shared among all siblings except two crop lands and the crop land which is given by my relative to my mother.We all decided to give some crop land and some money to my mother and among all my siblings who ever takes care of mother they can take the given land and money which is given while we siblings were sharing. One of my sibling(brother)taken care of my mother, and 4 years back my mother passed away. Now the another son is asking his share from what ever properties are left without distribution and asking share from the crop land which is given to my mother from my relative. But now the brother (who took care of my mother) is saying my mother without knowing any one she gave my relatives land to his son(grand son) and now they are not ready to give share from my relatives land. Now what should do to get share from relatives crop land?