Did the father err when he refused to sell a plot of land to the questioner's brother to marry him to it, and only wrote to him half of the marriage expenses, despite the brother's objection and his statement that this is forbidden and discriminatory, demanding that his father write to him what he wrote to his brother, even though the questioner spends on his parents and divorced sister, while his brother pays nothing?

Marrying off children is not part of the obligatory maintenance (nafaqa) due upon parents. If a father gives some of his children money to get married with, this is considered a benevolent act and a gift, not obligatory maintenance. The principle regarding gifts is to ensure equality among children, but it is permissible to differentiate in giving for a valid reason, such as the need of one of the children or a difference in their financial circumstances. Therefore, there is no fault on your father if he is unable to give you as much as he gave your brother.