1. Indian Registration Act was enacted in the year 1908 and Indian Stamp Act was enacted in the year 1899.
2. So, both the above Acts were applicable on the said purchase of land in your names which were required to be registered as per law to have legal enforceability.
3. Hence, the agreement, as stated by you in your query, executed in your and your brother's favour does not convey the title of the property to you in absence of registration of the title deed in your favour.
4. First you shall have to establish that the property belongs to you though it was not registered for which you can get the sale deed registered by the vendor or his legal heirs in your names.
5. If the property is registered in your and your brother's names, you can file a partition suit claiming your share on it.
6. In case your father has demised intestate, you have equall right with your siblings and mother on the properties left by your late father.