Yosef Karo

“ If he had a sexual relationship with his wife’s sister outside of marriage, and his wife and brother-in-law later died, there are those who say he would be prohibited from marrying his wife’s sister because just as the woman is prohibited to her husband, so too is she prohibited to the man she had an affair with, notwithstanding the fact that she was anyway prohibited to him. ”

“ Paragraph 25- One is permitted to marry his brother’s son’s wife and his sister’s son’s wife. One is permitted to marry his brother’s daughter and his sister’s daughter, and there is a mitzvah to do so, as discussed above in Siman 2.
Paragraph 26- One’s wife’s sister is biblically prohibited to him so long as his wife is alive, regardless of whether she is a paternal or maternal sister, even if he divorced his wife. After his wife dies, however, he is permitted to marry her sister. If a person grants a divorce, even because of a mere rumor, he becomes prohibited to the woman’s relatives. ”

“ Paragraph 27- If one was mekadesh a woman, and the woman went to another country and witnesses come and testify that she died, and the man married her sister, and it was later discovered that his first wife was still alive, he is prohibited to both women and both of them would need to receive a get from him. ”