Is Zakat due on an agricultural land that is owned but cultivated and benefited from by one's brother, and what is the amount of Zakat that must be paid if it is due?

There is no zakat due on you for this land. Zakat is only obligatory on what is produced from the land and fruits, under the well-known conditions, for its cultivator, the owner of the produce. Therefore, in this case, if the conditions for its obligation are met, the zakat is due on your brother, not on you.