Does the brother have the right to reclaim the plot of land that he won in the lottery and gratuitously relinquished to his sister, after learning that it is subsidized and that it was the only opportunity for him and his wife to acquire land, especially since the sister is in a better financial situation?

Your relinquishment of the right to own the land to your sister is a gift (hibah (definition)), and it is not permissible to revoke it, based on the Prophet, peace and blessings be upon him, who said: "He who takes back his gift is like a dog that swallows back its vomit." Your ignorance of whether the land was subsidized or a unique opportunity does not permit you to revoke your gift. However, your sister should treat you with kindness, just as you treated her, and she should offer to help you acquire land similar to the land you relinquished, in accordance with the Almighty's saying: "Is the reward for good [anything] but good?"