Is it permissible to pay zakat on wealth to treat a younger brother despite the father's ability and his claim of incapacitation?

The expenses for treating a young child are obligatory upon his capable father, and it is not permissible to use zakat to pay for his treatment in such a case. However, if the father is incapable or refuses, then it is permissible to use zakat for his treatment, and this might even be more meritorious for the reward of upholding kinship ties and giving charity.