1,812. Marrying a Missing Spouse's Sibling

Hilchos Geirushin 10:8

Similar to the previous halacha is the following case: Let’s say that one of two brothers betroths a woman. That brother leaves and it is later reported that he died, so the remaining brother performs yibum (levirate marriage), after which the first brother returns. In such a case, the woman must be divorced with a get from each of them and all the stringencies in the previous halacha apply. Similarly, if a man betrothed a woman and she then went abroad, after which it was heard that she died so he married her sister and it was later found that she didn’t die, he must divorce both of them and all the stringencies in the previous halacha apply.

Hilchos Geirushin 10:9

Let’s say that a man married a woman who then went abroad, he heard that she died so he married her sister, and it was then found that the first woman was still alive. The second woman doesn’t need a get and the first remains permitted to him. Similar rules apply to other women forbidden as prohibited relationships who were married under the misimpression that it was permitted and it was later found that the prohibition still applied. Such women do not need to be divorced because marriage can’t be effected with a woman who is one of the prohibited relationships.