Is the brother entitled to demand payment for the modern construction he completed, despite a prior agreement to share in its building?

If your brother built on your property at your command, using his own money, then the costs are a debt upon you. If he built without your command but for your benefit and intended to reclaim the expenses, then he is entitled to what he spent. However, if he built on your property for himself, it is usurpation (ghasb). Scholars differ on the ruling regarding buildings or plantings on usurped land:

Shafi'is and Hanbalis: They must be demolished and uprooted, and the usurper owes fair rental value and compensation for any damage. Malikis: The owner has the choice between demolition or paying the value of the building and plantings as dismantled. Hanafis: If the value of the land is greater, it is demolished; otherwise, it is not demolished, and the owner takes the value. Some Hanafis prefer that the owner takes the value in all cases unless the continued presence of the building causes him harm, and the value should be the actual value of the building at the time it was built.