Agreement of understanding on white paper valid?

Sir We are three brothers. Our brother in law n sister gifted a flat to all the three brothers. They don't have any children. As I will stay far away I could not attend at the time of registration n the registration is done on the other two brothers. Though my brother in law and sister purchased the flat but is purely registered directly on my two brothers only. There is no where mentioned in the document it is gifted by my brother in law and sister. And also not made any gift deed. It is purely done on trust my Brother in law and sister had on we brothers. Later my two brothers wrote an agreement of understanding on a white paper crystal clearly stating that i too have equal share in that flat on par with them and also mentioned registered document number in the agreement. This agreement is done in front of my brother in law and sister. They both signed the agreement. But please note due to trust in them I didn't registered the white paper document. Sir My humble questions sir 1) now due to our family situations and other lot of changes in our families, I asked my brothers to include my name in the registered document n I will bear the expenses. One brother accepted n the other strictly said he will never give share n never accept for my name to be included in the registered document. Please guide me what I have to do to get my share? 2) now can I register the agreement on white paper n file a case? 3) please guide me what procedures do I have to acquire my share? (Now This is the most important wish of my brother in law and sister as they don't have any child n I am talking care of them n they became old) Sir please note the builder of the apartment still alive n he knows the payment for the flat is given by my brother in law and sister )