Parashas Acharei Mos · Passuk 18:16
Chabad.org — English translation + Rashi
Torah Temima points to Yevamos 55a.
English: “It is your brother’s nakedness” — she remains forbidden even after the brother’s death when the brother was only a maternal half-brother (no yibbum).
Maternal brother’s wife after death
Yibbum applies only to a paternal brother’s widow; a maternal brother’s widow stays forbidden permanently.