1)
Which "Eishes Achicha" is the Pasuk referring to?
1.
Rashi in Yevamos, 2a: It incorporates a paternal and a maternal brother (the former provided he died leaving behind a child - Ki Seitzei Devarim, 25:5). 1
1
It applies whenever Yibum does not apply to his brother's wife, e.g. a maternal brother, the husband died before his brother was born, he is a Seris or she is an Ailonis (they are sterile)? (PF).