Answered by Ustadha Shaista Maqbool
Question: Assalamu alaikum wa rahmatullah,
What is the wisdom for why a man can’t marry his wife’s sister?
Answer: Wa’alaikum assalaam warahmatu Allah,
Allah, Most High, says in the Qur’an:
“Forbidden unto you are your mothers, and your daughters, and your sisters, and your father’s sisters, and your mother’s sisters, and your brother’s daughters and your sister’s daughters, and your foster-mothers, and your foster-sisters, and your mothers-in-law, and your step-daughters who are under your protection (born) of your women unto whom ye have gone in – but if ye have not gone in unto them, then it is no sin for you (to marry their daughters) – and the wives of your sons who (spring) from your own loins. And (it is forbidden unto you) that ye should have two sisters together, except what hath already happened (of that nature) in the past. Lo! Allah is ever Forgiving, Merciful.” [Al-Nisaa:23]
So Allah subhanahu wa ta’ala mentions at the end of this verse that it is prohibited to join between two sisters in marriage. This is the primary reason why it’s not allowed: simply because Allah says so.
The wisdom behind the prohibition, as the scholars have mentioned, is that the relationship between sisters is a very close relationship that necessitates a strong connection, deep care and honor for each other. Hence, for one to be a co-wife to her sister would be a cause for alienation and strong aversion to one another, and thus, it is seemly that this be prohibited.
And Allah knows best.
Checked & Approved by Faraz Rabbani