Question:
A man says he is Muslim by birth, and his wife converted from Hinduism. They are happily married with two children. His wife’s sister separated from her alcoholic husband and is now ready for remarriage. He asks whether he can invite her to Islam and marry her as a second wife?
Answer:
Islam generally prohibits marrying two sisters at the same time. A man cannot be married to both a woman and her sister simultaneously.
The Qur’an (4:23) clearly forbids marrying two sisters at the same time. However, if the first wife dies or the marriage ends through divorce, then marrying her sister becomes permissible.
There is also a hadith in Tirmidhi 1129; where a man accepted Islam while he was married to two sisters. The Prophetﷺ instructed him to keep only one and divorce the other. It was strongly prohibited to make the sisters turn towards each other as rivals.
Therefore, if you wish to help your wife’s sister, you can assist her in finding a suitable husband. But you cannot marry her while remaining married to her sister, because Islam clearly prohibits it.