All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The rich brother is not obliged to pay off the debt of his poor brother, regardless whether the debt is the result of a usury-based loan or otherwise. However, if he volunteers to pay his debt, it is a good act and a kind of maintaining the ties of kinship for which he would be rewarded, by the Permission of Allah, the Exalted.
Nonetheless, it is allowed for the rich brother to pay out the Zakat due on his money to his indebted brother who is unable to settle his debt. That is because a debtor is among one of the eight beneficiaries of Zakat. Rather, paying out Zakat to the indebted brother is even better than giving it out to others. This is evidenced by the Hadeeth of the Prophet in which he said, “Giving charity to a poor person is charity, and (giving) to a relative is two things, charity and upholding the ties of kinship.” [Ahmad, At-Tirmidhi, Ibn Maajah, An-Nasaa-i, etc.].
However, if the debtor used the money in au unlawful manner, he should not be given out from Zakat unless after showing repentance, which is the opinion we adopt here in Islamweb. You have stated that he – thanks to Allah – repented, and thus there is no harm in giving him out Zakat to repay his debt.
Allah knows best.