Since it was ancestral property, your brother was not entitled to alienate the property without your consent. The fact that the partition had not already taken place does not stand in the way of your interest as the co-owner of an undivided property.
Your brother is not entitled to sell more than the share to which he is entitled (i.e. 50%.) You should file a suit for partition as well as injunction in the civil court within the jurisdiction of which the property lies. I'd strongly recommend you to get in touch with an advocate for completing all the formalities.