Does an oath of divorce fall upon the wife if she informs her sister about a conversation that took place between her husband and his brother, knowing that she was already aware of that conversation before her husband told her about it and swore her to divorce not to tell anyone?

If you tell your sister or anyone else about your brother's call to your husband, a triple divorce will occur, leading to an irrevocable divorce (al-baynunah al-kubra) according to most scholars, even if your sister knew of your brother's desire for the call. The husband erred by disclosing your brother's secret, as it is a fundamental principle that gatherings are built on trust.