One of the most tragic and consequential sins committed is zina (adultery or fornication). This heinous crime destroys families and lives, and destroys in some people the hope of attaining Allah’s Pleasure. But in Islam, the door of repentance is always open, only that the repentance itself demands a resolve to cease from a sinful activity. A person who has engaged in an adulterous relationship must resolve to cease from the sinful sexual activity and return to Allah, seeking His Forgiveness. Allah will surely turn to him or her and shower him/her with His Mercy.
This indicates that the one who commits zina should never lose hope of gaining Allah’s Mercy. He or she should repent very seriously and immediately, rushing to Allah to seek His Forgiveness.
Allah says in the Qur’an: “Say: ‘O my Servants who have transgressed against their souls! Despair not of the Mercy of Allah: for Allah forgives all sins: for He is Oft-Forgiving, Most Merciful.” (Az-Zumar 53)
Moreover, the Glorious Qur’an gives glad tidings to those penitent people who have committed sins such as zina, regretted their sins, determined not to repeat them, and performed righteous deeds that Allah will replace their sins with hasanat or rewards. Allah says, “And true servants of Allah are those who cry not unto any other god along with Allah, nor take the life which Allah hath forbidden save in (course of) justice, nor commit adultery and whoso doeth this shall pay the penalty; the doom will be doubled for him on the Day of Resurrection, and he will abide therein disdained for ever; save him who repenteth and believeth and doth righteous work; as for such, Allah will change their evil deeds to good deeds. Allah is ever Forgiving, Merciful. And, whosoever repenteth and doeth good, he verily repenteth toward Allah with true repentance.” (Al-Furqan 68-71)
Tackling the issue you have raised concerning whether it is permissible for the man who committed zina with a certain woman to marry her daughter, we would like to clarify that the issue is debatable among Muslim jurists. Some of them state that such marriage is prohibited, whereas others state that it is permissible, but with certain guidelines.
To have a comprehensive view of the issue, we would like to cite for you the fatwa issued by the late Grand Sheikh of Al-Azhar Sheikh Jadul-Haqq `Ali Jadul-Haqq (may rest his soul in peace), in which he states the following:
“Muslim jurists differ concerning the legality of marriage to a woman with whose mother one has committed zina. The Hanafi` scholars, on the one hand, state that zina prohibits relationship by marriage, and hence it is not permissible for the one who committed zina with a certain woman to marry her daughter or mother, as the person who committed zina would be accustomed to have frequent contacts with the woman he had committed zina with, and most probably he would not stop such contact after marriage. Hence enmity would spread between the woman and her daughter or the woman and her mother.
The Shafi`i scholars, on the other hand, state that zina does not prohibit relationship by marriage, and according to Imam Shafi`i there is nothing wrong if the man marries the daughter of the woman with whom he committed zina.
Moreover, it is reported that some of the Maliki jurists prohibit such marriage and some others do not prohibit it.
I see that the opinion of Imam Shafi`i is preferable, and hence such marriages become permissible as long as the following conditions are observed:
1- If the man is quite sure that he will never approach the mother and tempt her to repeat the sin of zina after marrying her daughter.
2- If the man is sincere in his repentance and determined not to repeat the sin again, and he is quite sure about his will and repentance, then there is nothing wrong with the marriage and it is valid.
Having said this, I would like to stress that the opinion of the Hanafi` school is preferable if the above conditions are not observed so as to avoid enmity between the daughter and her mother as stated above.
Finally, I would like to stress that if a man committed zina with a certain woman and later concluded the marriage contract with her daughter, the woman with whom he committed zina before is like his real mother, and he is not allowed to marry her, let alone commit zina with her. If he committed zina with her, it would be as if he committed zina with his biological mother.”