Parashas Acharei Mos · Passuk 18:16
Chabad.org — English translation + Rashi
Torah Temima points to Yerushalmi Nedarim 3:2.
English: “Your brother’s wife” and “her yavam shall come upon her” were spoken in a single utterance.
Yibbum exception spoken together
The prohibition and its yibbum exception were legislated simultaneously; yibbum is not a later leniency but part of the original structure.