What is the Shar’i ruling on the reaction to knowing that a brother’s wife is accused of marital infidelity by letting a stranger into the house, without informing the husband for fear of the consequences?

One of the most important objectives of Islam is the preservation of the five necessities, among which is honor. You must not remain silent about your brother's wife's actions that arouse suspicion and tarnish your brother's honor. First, you should advise her, warn her against repeating what happened, and frighten her with Allah's punishment and public disgrace. If she does not respond, then threaten her with informing her husband. If she persists, you may inform her husband, provided that it does not lead to a greater harm. Be careful not to say what you have not witnessed or been reliably informed of, for speaking about people's honor is serious.