Parashas Acharei Mos · Passuk 18:18
Chabad.org — English translation + Rashi
Torah Temima points to Yevamos 97a.
English: Elsewhere the Torah says “lying”; here it says “taking” — to teach that the manner of marriage itself is forbidden.
Lekichah vs. shechivah
The prohibition targets the act of marrying the sister, not only the physical relationship.