Is it obligatory to inform the suitor about my addict brother's affliction, or is not informing him considered deceit, or is it a covering for my brother and a protection for my family?

If a young man of religion and good character proposes, you should agree to marry him, based on the Prophet's (peace be upon him) saying: "If someone whose religion and character you approve of proposes to you, then marry him." And since he is a man of religion, he will understand that a person is not held accountable for the misdeeds of another, as Allah Almighty said: "Every soul, for what it has earned, is retained," and "And no bearer of burdens will bear the burden of another." You are not required to inform him about your brother's condition, and not informing him is not deceit. Covering a Muslim's faults is religiously مطلوب (desirable/required), unless there is a stronger benefit in informing, which is not present here. Increase your supplications for your brother for guidance and righteousness.