Muqatil bin Sulayman (may Allah have mercy on him) said: Allah's saying "except what has already passed" [Surah al-Nisa: 22] refers to the wives of fathers, because people used to marry the wives of their fathers. Then He forbade (relations by) lineage and marriage and did not say "except what has already passed" there, because the Arabs did not marry within lineage and in-law relations. And concerning the two sisters He said "except what has already passed", because they used to combine them in their marriage; so the combining of them was forbidden, except what had already happened before the prohibition: "Indeed, Allah is ever Forgiving, Most Merciful." [Surah al-Nisa: 23] that is, regarding combining two sisters in one marriage before the prohibition.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13926