Is the money taken by the brother in exchange for watering the olive trees lawful or unlawful?

The brother is not entitled to exclusively claim the entire harvest on the pretext that he irrigated the land. Rather, he is entitled to the agreed-upon wage, or a customary wage if no agreement was reached. The produce belongs to the heirs of the landowner. If he refuses to return the heirs' right, they may forgive him or take the matter to court.