Is he who took out a loan in his name from the Agricultural Bank for his brother, and then it became clear to him that it was interest-bearing after he had appointed him as his agent and the loan had been disbursed, sinful?

Usury is one of the major sins. Your ignorance that the contract is usurious is an excuse for you before God, according to the Almighty's saying: (Our Lord, do not take us to task if we forget or make a mistake) [Al-Baqarah:286], and according to the Prophet's saying, peace and blessings be upon him: "Mistake, forgetfulness, and what they are coerced into have been lifted from my nation." You must فسخ (rescind/annul) the contract or withdraw your name from it if possible; otherwise, God does not burden a soul beyond its capacity. You must advise your brother and remind him of the gravity of his action and the necessity of repenting from it.