Does a wife's mumbling of a few words in front of her husband's sister, after she became aware of his adultery and pledged to conceal it, constitute divulging his secret and betraying her vow? And is expiation required for that?

If a woman utters these words unintentionally due to extreme anger, there is no sin upon her, based on the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts intended," and the Prophet's (PBUH) saying: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are coerced into."

However, if she utters them by choice, she may be sinful, because insinuation carries some of the meaning of explicit declaration. She must repent and not inform her husband, and she should frequently pray for his well-being. There is no specific expiation for this. It is important to warn against the danger of intimate companionship between men and women, especially for married individuals.