Is the plot of land that a father designated for his two sons, to the exclusion of their sisters, permissible, and are they (the sons) committing a sin religiously if they do not divide it with their sisters?

If a father designates something for his children but the gifted item is not taken possession of before the father's death, then the gift is not effective, and the gifted item becomes part of the estate. A father preferring some of his children over others without a valid reason is considered an injustice. If the children take possession of the gifted item during the father's lifetime, the gift becomes effective according to the majority of scholars, though it is recommended to return it to the estate to achieve justice. Some scholars have absolutely mandated its return. As for a gift given to an heir during a death-sickness, it is not effective unless the remaining heirs agree to it.