Is it permissible to give Zakat to a brother who is mentally ill and whose treatment costs have been borne by his siblings?

Zakat is obligatory on the wealth of a sick person if it reaches the nisab (threshold) and a hawl (one lunar year) passes over it, after deducting immediate debts. Their guardian should pay it on their behalf and give it to those who are eligible. As for the permissibility of giving zakat to a brother, that has been answered in Fatwa No. 161117.