Is it permissible for me to dispose of the plot of land that my father gifted me—while he is still alive—without my siblings, given that they are now demanding it, and is this gift considered a bequest that is impermissible?

It is not permissible for a father to prefer some of his children over others in giving gifts, unless the others consent. The Prophet, peace and blessings be upon him, called this an injustice. What a father bestows upon one of his sons is not considered legally valid. He must divide justly among his children if he intends to divide, otherwise it remains his property. The son must return the land to his father, and it is not permissible for him to take anything that aids in injustice.