How is zakat to be paid on money loaned to a brother, and on money that cannot be withdrawn from the bank due to current circumstances?

You must pay zakat on all your wealth, whether it is money you lent to your brother or money deposited in the bank. You can pay its zakat along with the rest of your wealth, or you can pay its zakat for past years when you receive it. It does not matter if the money in the bank is less than the nisab (minimum threshold) if it reaches the nisab when combined with the money lent to your brother. Nor does it matter that you cannot dispose of it, because it is similar to a debt owed by an insolvent person, and its zakat becomes due for past years when it is received.