What should I do, knowing that my paternal aunt's daughter, whom my grandmother chose as a wife for my brother, has committed adultery, given that I cannot inform any of my family members?

It should be known that a message received by mistake may be fabricated. If you confirm the authenticity of the message, there are two scenarios:

1. If the girl demonstrates change, righteousness, and uprightness, then there is no harm in your brother marrying her. She must be covered, as the Prophet, peace and blessings be upon him, said: "Whoever covers a Muslim, Allah will cover him on the Day of Resurrection." Repentance erases sin.

2. If she is not satisfactory in her religion and character, or if there is a possibility of her returning to what is forbidden, then it is obligatory to advise your brother not to marry her, as the Prophet, peace and blessings be upon him, said: "Religion is counsel (naseehah)." This should not be done by exposing her, but by diverting him from her through indirect means, or by informing him that you know something about her that necessitates turning away from proposing to her, even if you have to mention the content of the message without slandering her.