What is the prohibition regarding combining two sisters in marriage?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
Combining a woman with her sister (whether by kinship or fosterage, free or slave) is forbidden, according to the Quran, whether before or after consummation. If both are contracted simultaneously, the contract is void because neither has precedence. If one is married after the other, the first marriage is valid, but the second contract is void as it creates the forbidden combination. There is no scholarly disagreement on this rule.