Legally, since the Will which was the last one made by your father, had come into force upon his sad demise, the properties bequeathed in the Will shall devolve on the beneficiaries as per the recital made therein. Therefore your elder brother is right in denying your another brother a share which has not been entitled to him by your father in the Will.
The intestate agriculture property will devolve on all the five legal heirs of the deceased. Though it was agreed that the 50% share to be given to another brother in lieu of any other property, if the elder brother is not willing to this agreement, the matter to be decided legally. In the given situation, if you really interested to forego your share in the property and similarly if your mother and sister are ready and willing relinquish their rights on the property, all the three of you can jointly relinquish your rights to share in the said property by executing a registered release deed in favor of your another brother, after which, instead of original 50% agreed share, he may be benefited to 4/5th share in the property, discuss with your mother and sister about this and take a decision, you elder brother's consent need not be taken for this decision.