Parashas Acharei Mos · Passuk 18:14
Chabad.org — English translation + Rashi
Torah Temima points to Sanhedrin 28b.
English: “She is your aunt” — she is really the uncle’s wife; the Torah treats the wife as the husband, and from here one may not testify about a stepson’s wife.
Wife equated to husband (edut)
The verse both forbids the uncle’s wife and establishes that a wife is legally identified with her husband for certain kinship and testimony rules.