Rashi on Deuteronomy 25:6/רש"י על דברים כ״ה:ו׳

1והיה הבכור AND IT SHALL BE THE FIRSTBORN … — It is the eldest brother who is to perform the duty of levirate marriage (Sifrei Devarim 289:1; Yevamot 24a).

2אשר תלד WHOM SHE WILL BEAR — This excludes from the operation of this law a sterile woman, since she cannot bear children (Sifrei Devarim 289:2).

3יקום על שם אחיו HE SHALL SUCCEED IN THE NAME OF HIS BROTHER — he who has married his (the deceased brother’s) wife shall receive the portion of the dead brother in his father’s property (Sifrei Devarim 289:3).

4ולא ימחה שמו THAT HIS NAME BE NOT WIPED OUT [OF ISRAEL] — thus excluding from the levirate marriage the wife of a castrate, whose name is anyhow wiped out (Sifrei Devarim 289:5; Yevamot 24a).