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).
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with the wife of his brother is liable excision if he was volitional; and if he was inadvertent, he is obligated to bring a fixed sin-offering. And this is after the death of his brother and he left children, or after he divorced her. As if it was in his life when she is still [with] him, it is well-known that the law of the one who lays with the wife of his brother (it should say, with a married woman) [is that] his death is with strangulation. And the prohibition of the wife of the brother leaves no room to prohibit secondary [prohibitions].
עֶרְוַת אֵשֶׁת אָחִיךָ לֹא תְגַלֵּה – שֶׁלֹּא לָבוֹא עַל אֵשֶׁת אָחִיו, שֶׁנֶּאֱמַר (ויקרא יח טז) עֶרְוַת אֵשֶׁת אָחִיךָ לֹא תְגַלֵּה. וְאֶחָד אָחִיו מֵאָבִיו לְבַד אוֹ מֵאִמּוֹ לְבַד, וּבֵין מִן הַנִּשּׂוּאִין וּבֵין מִן הַזְּנוּת אִשְׁתּוֹ עֶרְוָה עָלָיו, שֶׁאָחִיו נִקְרָא מִכָּל מָקוֹם. וּמִכֵּיוָן שֶׁאֵרְסָהּ נִקְרֵאת אֵשֶׁת אָח וַאֲסוּרָה, וְכָל שֶׁכֵּן אַחַר שֶׁנְּשָׂאָהּ, וּבֵין בְּחַיֵּי אָחִיו וּבֵין אַחַר מוֹתוֹ הִיא אֲסוּרָה עָלָיו מִשּׁוּם אֵשֶׁת אָח, חוּץ מֵאֵשֶׁת אָח שֶׁלֹּא הִנִּיחַ בֵּן שֶׁהַתּוֹרָה צִוְּתָה עַל זוֹ בְּפֵרוּשׁ שֶׁיִּשָּׂאֶנָּה הָאָח, וְזֹאת הִיא מִצְוַת יִבּוּם כְּמוֹ שֶׁנְּפָרֵשׁ בְּעֶזְרַת הַשֵּׁם (מצוה תקצח).
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).
וְנוֹהֵג אִסּוּר זֶה בְּכָל מָקוֹם וּבְכָל זְמַן. וְהָעוֹבֵר עָלֶיהָ וְשָׁכַב אֶת אֵשֶׁת אָחִיו חַיָּב כָּרֵת אִם הָיָה מֵזִיד, וְאִם שָׁגַג חַיָּב לְהָבִיא חַטָּאת קְבוּעָה. וְזֶה יִהְיֶה אַחַר מוֹת אָחִיו וְנִשְׁאֲרוּ לוֹ בָּנִים, אוֹ אַחַר שֶׁגֵּרְשָׁהּ, שֶׁאִלּוּ בְּחַיָּיו בְּעוֹדָהּ תַּחְתָּיו, יָדוּעַ הוּא שֶׁדִּין הַשּׁוֹכֵב אֶת אֵשֶׁת אִישׁ מִיתָתוֹ בְּחֶנֶק. וְאֵין בְּאִסּוּר אֵשֶׁת אָח מָקוֹם לְאִסּוּר שְׁנִיּוֹת.
And this prohibition is practiced in every place and at all times. And one who transgresses it and laid with the wife of his brother is liable excision if he was volitional; and if he was inadvertent, he is obligated to bring a fixed sin-offering. And this is after the death of his brother and he left children, or after he divorced her. As if it was in his life when she is still [with] him, it is well-known that the law of the one who lays with the wife of his brother (it should say, with a married woman) [is that] his death is with strangulation. And the prohibition of the wife of the brother leaves no room to prohibit secondary [prohibitions].