Parashas Acharei Mos · Passuk 18:18
Chabad.org — English translation + Rashi
Torah Temima points to Yevamos 3b / 13a.
English: “To rival her” — parallel to yibbum’s “upon her” limits the case; “to rival” expands to include co-wives and their co-wives in the chain of exclusion from yibbum.
Tzarot excluded from yibbum
A rival wife (tzarah) of a forbidden relative is also barred from yibbum, and the exclusion chains outward.