Is it permissible for a son to allocate a portion of his Zakat to contribute to the treatment costs of his sister, who is mentally ill, even though the father is still capable of bearing these costs alone?

It is not permissible for a son to give his Zakat (definition) wealth to his father for him to spend on his daughter, because the father is capable of providing for her, and his daughter is considered wealthy due to the obligation of her maintenance upon him. Charity is not permissible for the wealthy or for the strong, able-bodied earner. However, the son can donate money to help his father with his sister's treatment as an act of charity and to maintain kinship ties.