What should a woman do if she learns about a relationship between a married woman and a man who is not her husband, and she is related to the married woman's husband?

One must ascertain that an illicit relationship exists before accusing the wife of infidelity. If it is proven, she should be reminded of Allah and advised. If she persists, her family should be notified first to deter her. If that does not work, or if she does not have influential family members, then her husband must be informed so he can consider what is most appropriate.