Allah said,

﴿وَأَن تَجْمَعُواْ بَيْنَ الاٍّخْتَيْنِ إَلاَّ مَا قَدْ سَلَفَ﴾

(...and two sisters in wedlock at the same time, except for what has already passed;) The Ayah commands: you are prohibited to take two sisters as rival wives, or rival female-servants, except for what had happened to you during the time of Jahiliyyah, which We have forgiven and erased. Therefore, no one is allowed to take or keep two sisters as rival wives, according to the consensus of the scholars of the Companions, their followers, and the Imams of old and present. They all stated that taking two sisters as rival wives is prohibited, and that whoever embraces Islam while married to two sisters at the same time is given the choice to keep one of them and divorce the other. Imam Ahmad recorded that Ad-Dahhak bin Fayruz said that his father said, "I embraced Islam while married to two sisters at the same time and the Prophet commanded me to divorce one of them.''

﴿وَالْمُحْصَنَـتُ مِنَ النِّسَآءِ إِلاَّ مَا مَلَكْتَ أَيْمَـنُكُمْ كِتَـبَ اللَّهِ عَلَيْكُمْ وَأُحِلَّ لَكُمْ مَّا وَرَاءَ ذَلِكُمْ أَن تَبْتَغُواْ بِأَمْوَلِكُمْ مُّحْصِنِينَ غَيْرَ مُسَـفِحِينَ فَمَا اسْتَمْتَعْتُمْ بِهِ مِنْهُنَّ فَـَاتُوهُنَّ أُجُورَهُنَّ فَرِيضَةً وَلاَ جُنَاحَ عَلَيْكُمْ فِيمَا تَرَاضَيْتُمْ بِهِ مِن بَعْدِ الْفَرِيضَةِ إِنَّ اللَّهَ كَانَ عَلِيماً حَكِيماً ﴾

(24. Also (forbidden are) women already married, except those (slaves) whom your right hands possess. Thus has Allah ordained for you. All others are lawful, provided you seek them (with a dowry) from your property, desiring chastity, not fornication. So with those among them whom you have enjoyed, give them their required due, but if you agree mutually (to give more) after the requirement (has been determined), there is no sin on you. Surely, Allah is Ever All-Knowing, All-Wise.)