What should the questioner do upon discovering that her brother is bringing a married woman into their home, knowing that her mother is aware but has not taken a stand, and fearing Allah's reckoning upon her mother for her silence?

You must forbid your brother from this evil and warn him of the consequences of disobedience. If he does not respond, then inform someone who can prevent him. As for your mother, it is not permissible to abandon her or mistreat her. The duty is to honor her and be good to her. If she falls short in forbidding your brother, advise her gently and politely, without harshness or excessive persistence, because the right of the mother is great, and enjoining good and forbidding evil with her is not like with others. Ahmad said: If one sees his father doing something disliked, he should inform him without harshness or mistreatment, and not be rude in speech to him; otherwise, he should leave it.