What is the fate of my brother's business, which he started eight years ago with an interest-based loan, unaware of its prohibition, knowing that he used to lend money and medicine to his villagers without interest and helped many?

As long as your brother was ignorant of the prohibition of the usurious loan he took from the bank, then there is nothing upon him, and his trade is permissible for him, according to the words of Allah Almighty: "So whoever receives an admonition from his Lord and stops, then he may keep what he has already attained, and his affair is up to Allah." However, he is obliged to cease such transactions once he learns of their prohibition.