To what extent is a verbal will issued by a gravely ill person, appointing one of his sons as guardian over his adult, sane, and mature siblings, valid and obligatory to implement?

If a father designates one of his sons to be a guardian over his adult, رشيد (of sound judgment) siblings, this designation is not valid and should not be implemented. This is because it is a designation for interdicting an adult of sound judgment, and interdiction upon such a person is only permissible for a valid reason and requires a ruling from a judge. However, if a father designates his son to arrange the marriages of his daughters, this is permissible, and the son becomes their guardian for marriage if he is qualified for guardianship.