What is the ruling on marrying a girl with whom the husband had an illicit relationship, knowing that he previously had an illicit relationship with her sister, and that this girl has a daughter from a previous illegitimate relationship and an invalid marriage? And do previous illicit relationships affect the validity of marriage after repentance?

Adultery is a grave crime, but whoever repents, Allah accepts their repentance. The marriage of an adulterer to an adulteress after repentance is valid. Marriage before repentance is forbidden, based on the words of the Almighty: (The adulterer does not marry except an adulteress or a polytheist, and as for the adulteress, none marries her except an adulterer or a polytheist, and that has been made unlawful for the believers). If your wife had entered into an invalid marriage, your marriage is valid if she was divorced or her invalid marriage was annulled before your contract with her.

If a man commits adultery with a woman and her 'iddah (waiting period) has expired, her sister is not forbidden to him by agreement, and what is prohibited is combining two sisters in marriage. As for the mother or daughter of the woman with whom adultery was committed, scholars have differed: - The Hanafis and Hanbalis held the view of prohibition, considering illicit intercourse to be like lawful intercourse in establishing the حرمة المصاهرة (prohibition of affinity). - The Malikis and Shafi'is held the view of non-prohibition, which is the more preponderant opinion, because what is unlawful does not render lawful things unlawful.

As long as your marriage took place after a period in which the adulteress's 'iddah had expired, your marriage is valid. Dispel the obsessive doubts, hold fast to your wife, and remain steadfast in obedience to Allah.