Is it permissible for siblings to demand their share of their father's money from their brother, who studied medicine, for what the father paid?

If the expenses for studying medicine are necessary for the brother, and he is unable to provide them, then the father is obligated to spend on them to the extent of the son's need. If the father does not do so and dies, then the other children have the right to demand that their brother share with them what he was favored with, because favoring one child is an injustice that must be rectified during the father's lifetime or after his death, according to the more sound of the two opinions. As for before the father's death, the brother must return the favor to his father.