Is zakat obligatory on a loan amount that has been outstanding for years, has reached the nisab, and a full hawl has passed over it?

The debt owed to your brother has two scenarios:

1. If the debtor acknowledges the debt and is able to pay it, then zakat is obligatory on it every year. Your brother may pay its zakat annually along with his own wealth, or he may defer its zakat until he collects it, then pay its zakat for all the past years. 2. If the debtor is insolvent or denies the debt, then its zakat is not obligatory except after he collects it. He then pays its zakat for one year or for all the past years, and this latter option is closer to piety.