Parashas Acharei Mos · Passuk 18:18
Chabad.org — English translation + Rashi
Torah Temima points to Kiddushin 50b.
English: One who betroths a woman and her daughter, or a woman and her sister, simultaneously — neither is betrothed; what cannot take effect sequentially cannot take effect simultaneously.
Simultaneous kiddushin invalid
Attempted concurrent betrothal of two mutually forbidden relatives creates no marriage bond.