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The commandment of yibum (levirate marriage), allows the wife of a man who dies without offspring to be married by one of his surviving brothers, negating the Biblical prohibition forbidding a man from marrying his brother’s wife.
What is the status of their relationship, once yibum has been performed? Are they still seen as connected because of the original marriage to the deceased brother, or are they simply a married couple?
The biblical prohibition applies not only to a person who marries two sisters, making them into permanent rivals - for after he marries one sister, the other is forbidden to him, and any "marriage" to the second does not take effect.
The verse furthermore prohibits any relations with his wife's sister - even a one-time affair, as stated in the conclusion, "to uncover her nakedness beside the other (his wife)." But the reason for the prohibition is, as Ramban teaches, "For it is not proper that one take a woman and her sister (as wives), making them into rivals, for they should love one another and not distress each other."
We explore the notion of rival sisters and how levirate marriage differs from sororate marriage across cultures.
And the curious case of Chief Rabbi Nathan Adler’s contribution to the 1849 Victorian commission into the some 13000 illegal levirate marriages in Britain which eventually was codified into the The Deceased Wife’s Sister’s Marriage Act of 1907