What is the ruling on a man marrying a woman with whom he committed adultery after their repentance, and does his agreement to marry her, while she is his wife's sister before their divorce, constitute combining two sisters, thereby making her unlawful for him, and is this agreement considered void after he frankly told her of his unwillingness to marry her?

A husband accusing his wife falsely is a grave sin, based on the Almighty's saying: "And those who accuse their wives and have no witnesses except themselves - then the testimony of one of them shall be four testimonies [swearing] by Allah that he is of the truthful. And the fifth [oath will be] that the curse of Allah be upon him if he should be among the liars." [An-Nur: 6-7]. It is not permissible to accuse a wife of adultery unless one sees her committing adultery or has a confirmed suspicion, such as her confession or repeatedly seeing a man with her in a suspicious place. It is better to cover her fault and divorce her if he dislikes her, provided there is no child he would disown. Sincere repentance from adultery is obligatory, and it is permissible for the adulterer to marry the woman he committed adultery with after repentance, according to the consensus of the four schools of thought. As for before repentance, it is permissible according to the majority of scholars but forbidden according to the Hanbalis. An agreement to marry the wife's sister, whether before or after divorcing her, has no legal value and no ruling follows from it, as it is a promise that does not necessitate fulfillment.