Is it permissible to work for someone who started a business project with a bank loan to help his brother, and is the money earned from this work unlawful?

The borrower from a usurious bank must be advised that their act is forbidden by Islamic law and is precisely the usury for which Allah has cursed its perpetrator. They must offer sincere repentance by returning what they have taken and ridding themselves of it, and seek what is lawful, enduring patience even if it is little, for sustenance is decreed and apportioned. The Messenger of Allah, peace and blessings be upon him, said: "Indeed, no soul will die until it completes its sustenance, so fear Allah and be graceful in seeking it. And let not the delay in obtaining sustenance lead you to seek it through disobedience to Allah, for Allah's bounty cannot be attained except through His obedience." As for working with them, it is preferable to avoid it, and to be cautious about accepting their gifts, eating from their wealth, and engaging in all other financial dealings.