Is zakat obligatory on a savings account designated for a mentally disabled brother, intended to provide him with a monthly allowance, knowing that his income is not stable?

What a disabled person possesses is subject to zakat (definition) on their wealth if it reaches the nisab (definition) and a full year has passed over it. It is not permissible to place money in a savings account, and usury is not subject to zakat; rather, it must be disposed of by spending it on general Muslim welfare or for the poor and needy. It is permissible to give zakat to a brother if he is poor, but not permissible if he is not poor, even if he is disabled.