My Eldest brother does'nt want to proceed with Partition deed

We are 3 brothers and have our father expired in oct'14. Now we are going for partition deed but our eldest brother is not showing any interest in proceeding for partition deed. There is no Will made by my father and my eldest brother was given the pass books of some cultivating land which he can put for sale and sold one land already by taking advance money. Registration is not done and he is asking my mother's sign so that to complete registration and get full money from the party. Actually my brother's land area is not as per the passbook, when a lawyer checked in their books in process of preparing for partition deed. To make the partition deed, lawyer asked to rectify the passbook so that he can make everything ready for partition deed. My brother is not trying to rectify the land area and instead he is willing to get the sign of my mother so that he can get remaining money from the party. Kindly help how to proceed legally. My mom is saying "until the partition deed is made, she will not sign for any sale agreements" We have Legal heir document.