Is it permissible to give zakat on wealth to a needy brother for a surgical operation, if the father is able to cover the costs from his end-of-service gratuity but has no other income?

It is permissible to give Zakat to a sick brother for his medical expenses if he is unable to bear the costs of the operation, provided he is an adult and his sustenance is not obligatory upon a capable father. However, if his sustenance is obligatory upon a capable father, then it is not permissible to give him Zakat.