[Machine] Imam al-Shafi'i, may Allah have mercy on him, said in his book al-Rada'a that the eldest son of a man used to be inherited by his father's wife. The man used to marry both sisters at the same time. However, Allah, the Most High, prohibited anyone from marrying two sisters at the same time or marrying what his father had married, except for what was done during the pre-Islamic era before they knew it was prohibited. It is not that he approved of what they had done before Islam came into their hands.  

قَالَ الشَّافِعِيُّ رَحِمَهُ اللهُ فِي كِتَابِ الرَّضَاعِ كَانَ أَكْبَرُ وَلَدِ الرَّجُلِ يُخَلَّفُ عَلَى امْرَأَةِ أَبِيهِ وَكَانَ الرَّجُلُ يَجْمَعُ بَيْنَ الْأُخْتَيْنِ فَنَهَى اللهُ تَعَالَى عَنْ أَنْ يَكُونَ أَحَدٌ مِنْهُمْ يَجْمَعُ فِي عُمْرِهِ بَيْنَ أُخْتَيْنِ أَوْ يَنْكِحُ مَا نَكَحَ أَبُوهُ إِلَّا مَا قَدْ سَلَفَ فِي الْجَاهِلِيَّةِ قَبْلَ عِلْمِهِمْ بِتَحْرِيمِهِ لَيْسَ أَنَّهُ أَقَرَّ فِي أَيْدِيهِمْ مَا كَانُوا قَدْ جَمَعُوا بَيْنَهُ قَبْلَ الْإِسْلَامِ  

[Machine] "Allah ﷻ says {except for what has already occurred} [Quran 4:22], meaning in regards to marrying the wives of one's fathers, because the Arabs used to marry their fathers' wives. Then Allah prohibited this and also prohibited marrying one's female relatives through marriage or blood. He did not say 'except for what has already occurred' concerning marrying relatives through blood and marriage, and he said in regards to two sisters {except for what has already occurred} [Quran 4:23], because they used to marry both sisters together. So he prohibited marrying both sisters together, except for what has already occurred before the prohibition. {Indeed, Allah is Forgiving and Merciful} [Quran 4:23] because it was done before the prohibition of marrying both sisters together."  

اللهُ ﷻ {إِلَّا مَا قَدْ سَلَفَ} [النساء 22] يَعْنِي فِي نِسَاءِ الْآبَاءِ لِأَنَّ الْعَرَبَ كَانُوا يَنْكِحُونَ نِسَاءَ الْآبَاءِ ثُمَّ حَرَّمَ النَّسَبَ وَالصِّهْرَ وَلَمْ يَقُلْ إِلَّا مَا قَدْ سَلَفَ لِأَنَّ الْعَرَبَ كَانَتْ لَا تَنْكِحُ النَّسَبَ وَالصِّهْرَ وَقَالَ فِي الْأُخْتَيْنِ {إِلَّا مَا قَدْ سَلَفَ} [النساء 22] لِأَنَّهُمْ كَانُوا يَجْمَعُونَ بَيْنَهُمَا فَحَرَّمَ جَمْعَهُمَا جَمِيعًا إِلَّا مَا قَدْ سَلَفَ قَبْلَ التَّحْرِيمِ {إِنَّ اللهَ كَانَ غَفُورًا رَحِيمًا} [النساء 23] لِمَا كَانَ مِنْ جِمَاعِ الْأُخْتَيْنِ قَبْلَ التَّحْرِيمِ  

كَانَ إِذَا تُوُفِّيَ الرَّجُلُ فِي الْجَاهِلِيَّةِ عَمَدَ حَمِيمُ الْمَيِّتِ إِلَى امْرَأَتِهِ فَأَلْقَى عَلَيْهَا ثَوْبًا فَيَرِثُ نِكَاحَهَا فَيَكُونُ هُوَ أَحَقُّ بِهَا فَلَمَّا تُوُفِّيَ أَبُو قَيْسِ بْنُ الْأَسْلَتِ عَمَدَ ابْنُهُ قَيْسٌ إِلَى امْرَأَةِ أَبِيهِ فَتَزَوَّجَهَا وَلَمْ يَدْخُلْ بِهَا فَأَتَتِ النَّبِيَّ ﷺ فَذَكَرَتْ ذَلِكَ لَهُ فَأَنْزَلَ اللهُ فِي قَيْسٍ {وَلَا تَنْكِحُوا مَا نَكَحَ آبَاؤُكُمْ مِنَ النِّسَاءِ إِلَّا مَا قَدْ سَلَفَ} [النساء 22] قَبْلَ التَّحْرِيمِ حَتَّى ذَكَرَ تَحْرِيمَ الْأُمَّهَاتِ وَالْبَنَاتِ حَتَّى ذَكَرَ {وَأَنْ تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ} [النساء 23] قَبْلَ التَّحْرِيمِ {إِنَّ اللهَ كَانَ غَفُورًا رَحِيمًا} [النساء 23] فِيمَا مَضَى قَبْلَ التَّحْرِيمِ