Imam al-Shafi'i (may Allah have mercy on him) says in Kitab al-Rada' (the Book of Nursing) that a man's eldest son, after his father's death, (would take his place with regard to the wife), that is, he would marry his father's wife; and a man would keep two sisters in one marriage. So Allah Almighty forbade that anyone should combine two sisters in marriage during his lifetime or marry his father's wife, except what had already happened in the Jahiliyyah before they knew of its prohibition; but now even those who had done so before Islam should not keep two sisters together.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13925