Is it obligatory to inform the suitor that the brother smokes hashish, knowing that he will eventually find out, and is this considered exposing the brother?

You are not required to inform the fiancé about your brother's hashish smoking, and there is no deception in not informing him. A brother's wrongdoing is not a defect that must be disclosed during an engagement, and the principle is that a person is not blemished by the wrongdoing of another. Strive to advise your brother, and supplicate frequently for his guidance.