Halakhah su Levitico 18:16

The nakedness of the wife of your brother you shall not reveal: To not have intercourse with the wife of your brother, as it is stated (Leviticus 18:16), "The nakedness of the wife of your brother you shall not reveal." And it is one whether it is his brother from only his father or only his mother; and whether it is from marriage or whether from licentiousness - his wife is a sexual prohibition for him, since he is called, "his brother," regardless. And from when he betrothed her, she is called, "the wife of his brother," and is forbidden - and all the more so after he marries her. And it is whether it is in the lifetime of his brother or whether after he died, that she is forbidden to him on account of "the wife of the brother." [This is] except for the wife of the brother that did not leave a child, as the Torah commanded explicitly about that, that the brother should marry her. And that is the commandment of levirate marriage, as we will explain with God's help (Sefer HaChinukh 598).