⚖️ Polygyny: rulings & justice · #21
Besides two sisters, which other women may not be joined as wives?
The rule: two women who, if one of them were considered a man, would be forbidden to marry each other because of lineage or breastfeeding, may not be joined.
This includes two sisters (Surah an-Nisa' 4:23), a woman with her paternal or maternal aunt (Sahih al-Bukhari 5109; Sahih Muslim, Kitab an-Nikah), and a woman with her niece and so on along the same relationships, including through breastfeeding. Conversely, a woman and her cousin may be joined because cousins are not mahram. A mother-in-law and a stepdaughter fall into the category of permanent prohibition, not merely a prohibition on joining. See Mahram and Glossary.
This includes two sisters (Surah an-Nisa' 4:23), a woman with her paternal or maternal aunt (Sahih al-Bukhari 5109; Sahih Muslim, Kitab an-Nikah), and a woman with her niece and so on along the same relationships, including through breastfeeding. Conversely, a woman and her cousin may be joined because cousins are not mahram. A mother-in-law and a stepdaughter fall into the category of permanent prohibition, not merely a prohibition on joining. See Mahram and Glossary.
Sources
Surah an-Nisa' 4:23Sahih al-Bukhari 5109Sahih Muslim, Kitab an-Nikah