نَا أَبُو بَكْرٍ ، نَا الرَّبِيعُ بْنُ سُلَيْمَانَ ، وَأَبُو إِبْرَاهِيمَ الْمُزَنِيُّ ، قَالا : عَنِ الشَّافِعِيِّ ، قَالَ : " إِذَا أَسْلَمَ وَتَحْتَهُ أُخْتَانِ خُيِّرَ أَيَّهُمَا شَاءَ ، فَإِنِ اخْتَارَ وَاحِدَةً ثَبَتَ نِكَاحُهَا ، وَانْفَسَخَ نِكَاحُ الأُخْرَى ، وَسَوَاءٌ كَانَ نَكَحَهُمَا فِي عُقْدَةٍ أَوْ فِي عَقْدٍ " .
Imam Shafi'i says: When a person becomes Muslim and his wives are real sisters, then that person may choose whichever one of the two he wishes, and his marriage with that woman will remain valid, while his marriage with the other woman will be annulled. In this regard, it will make no difference whether (before accepting Islam) he married both of them at the same time or one after the other.