How should I deal with the money of my brother's orphaned children? Is zakat due on it? And who has the right to make decisions regarding this money, especially since they own a bank deposit that matures at the end of the year?

The amount must be withdrawn from the usurious bank. The principal amount belongs to your brother's heirs, while the interest should be spent on charitable causes. It is permissible to spend it on your brother's children if they are in need. Zakat is obligatory on the principal amount every year if each heir's share reaches the nisab. The guardian is the one who decides matters related to your brother's children, and he is not permitted to dispose of the minor's wealth except in what is most beneficial for him.