[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