Is it permissible to visit a brother who lives with a woman who is a stranger to him without marriage?

If the woman is a stranger to your brother (i.e., not a mahram), then his action is a great sin and disobedience. You must change this evil to the best of your ability. If you cannot, then advise him. If he persists in his action, then abandon him if that would deter him from disobedience.