How will we reconcile the prohibition against marrying one's brother's wife with the Mitzvah of Yibum?
Targum Yonasan: The Mitzvah of Yibum applies only where one's brother has died and left behind no children from the Yevamah, whereas the current Pasuk is referring specifically to where he is still alive or where the Yevamah has children from him.
What are the connotations of "Nidah hi"? Why does the Torah insert it?
Rashi #1: It means that such a Bi'ah is rejected and despicable.
Rashi #2 (citing Yevamos, 54b): To incorporate Ha'ara'ah in the Isur, like by a Nidah. 1
Sifra #1: To teach us that the Pasuk is speaking about the wife of one's paternal brother, who, like a Nidah, is sometimes Mutar (where the brother died without leaving children) and sometimes Asur - whereas the wife of a maternal brother is always permitted. 2
Sifra #2: To teach us that, like a Nidah who is Chayav Kareis whilst she is Tamei even though she will become permitted later, Eishes Achiv is Chayav Kareis during her hiusband's lifetime even though she will beome permitted after his death.
Why is this necessary, seeing as all the Arayos are learned from Nidah via a Hekesh. Refer to 20:18:1.1:1*.
See Torah Temmah, note 53.
Rashi (17:9) writes that Ariri (being childless) applies to all the Kerisos. If so, why is Eishes Achiv repeated? We already know that there is Kareis from 18:29?
Moshav Zekenim (in Pasuk 17): Refer to 20:21:2:2 and note.
Why does the Torahy write "Nidah hi" specifically by Eishes Ach?
Moshav Zekenim: Because Eishes Ach can become permitted (in a case of Yibum), just like a Nidah becomes permitted to her husband. 1
Also Achos Ishto and Eishes Ish can become permitted, but those are merely permitted. When Ishto Nidah and Eishes Ach become permitted, there is a Mitzvah to be intimate with them (PF).
