What is the legitimate Islamic (Sharia) opinion regarding a mother's insistence on selling a piece of land, whose value is five times the debt owed for gold, to the questioner's sister, instead of mortgaging it until the debt is paid, out of consideration for the rest of the siblings?

It is permissible to sell land to a sister in exchange for gold. However, since the price of the land exceeds the debt by five times, this is considered a sale of favor (bay' muhabah), which is a type of gift. Parents must be just among their children in giving gifts, and it is not permissible to prefer some over others unless there is a legitimate reason for it. Therefore, it is not permissible for the mother to favor her daughter with this land.