What is the ruling on informing a husband of a suspicion of an illicit relationship between him and another woman? Is this considered slander if it is not proven? And what can be done to keep this woman away from the husband?

The Shari'ah warns against a woman being lenient with her husband's relatives, as the Prophet, peace and blessings be upon him, said: "Beware of entering upon women." A man asked: "O Messenger of Allah, what about the Hamw (male in-law)?" He replied: "The Hamw is death." A wife's request to her husband to be cautious in his interactions with his brother's wife, and her informing him of what she finds suspicious in her actions with him, is a correct act and not forbidden slander, as long as the suspicion is based on observable actions and not mere doubts.