What should an individual from the family do if they discover an ongoing illicit relationship between their sister's husband and their brother's wife? Should they advise speaking with them or remain silent, fearing the consequences of both actions?

What transpires between a sister's husband and his wife's sister-in-law is a grave abomination, caused by deviation from religious law, mixing, and private seclusion. The Prophet ﷺ warned against entering upon women, especially the "hamw" (male in-law), because it is death.

This abomination must be changed: whether you are able to do so by hand, by tongue, or by heart.

It is advisable to speak with the man and your brother’s wife to abandon this abomination, threatening to inform the family if they do not desist. If they repent, their secret should be kept. If they do not desist, the woman's husband should be alerted to pay attention to his wife, without explicit disclosure, to avoid false accusation (qadhf).

It is not permissible to deny the lineage of a child born to the husband of the mother as long as she is his lawful bed (firaash), for the Prophet ﷺ said: "The child is for the bed, and for the fornicator is the stone."