Commentary for Kiddushin 102:3
ואם אמר הראויה לביאה תתקדש לי אינה מקודשת ומעשה נמי בחמש נשים ובהן שתי אחיות וליקט אחד כלכלה של תאנים ואמר הראויה לי מכם תתקדש לי ואמרו חכמים אין אחיות מקודשות
But if he says: 'She of you who is eligible for intercourse, let her be betrothed unto me,' she is not betrothed.<span class="x" onmousemove="('comment',' For neither is eligible.');"><sup>3</sup></span> And thus IT ONCE HAPPENED TO FIVE WOMEN, AMONG WHOM WERE TWO SISTERS, THAT A MAN GATHERED A BASKET OF FIGS AND SAID, 'She of you who is eligible [for intercourse], let her be betrothed unto me': THE SAGES THEN RULED: THE SISTERS ARE NOT BETROTHED, Raba reconciled it with his opinion: If a man betroths one of a woman and her daughter or a woman and her sister, it is as though he betrothed A WOMAN AND HER DAUGHTER OR A WOMAN AND HER SISTER SIMULTANEOUSLY, AND THEY ARE NOT BETROTHED.
Daf Shevui to Kiddushin
Clause 1: If he betroths two women simultaneously that cannot be betrothed consecutively, neither are betrothed. This follows Rabbah’s rule.
Deduction, part 1: If he betroths only one of these women, both are doubtfully betrothed. The kiddushin are essentially valid even though they cannot be followed by intercourse.
Deduction, part 2: If he explicitly says that he is betrothing only the one who is allowed to have sex with him, neither is betrothed. Since he does not know which one he is betrothing, neither of them can have sex with him.
Clause 2: This is exactly what the husband said. The sisters are not married not because “kiddushin that cannot be followed by intercourse are not valid” but because he explicitly stated that he was betrothing only those eligible for intercourse. Thus Abaye has resolved this clause with his opinion.