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If a man died childless, the yavam may not consummate the levirate marriage with either sister because it is possible that he will do so with the sister who was not betrothed to his brother, who is therefore forbidden to him as the sister of a woman who is bound to him by a levirate bond. This is in fact the case of the baraita, and the baraita teaches that she performs ḥalitza, even though she is not permitted to enter into levirate marriage.
The Gemara rejects the proof: How can these cases be compared? There, in the case of uncertainty, if Elijah were to come and say that the man betrothed this one of the sisters, then she would be eligible for ḥalitza and levirate marriage. It is clear, then, that fundamentally the sister who was betrothed is actually eligible for levirate marriage, and it is merely a lack of knowledge that prevents her from entering into it.
However, here, with regard to a woman during the first three months following her previous marriage, if Elijah were to come and say that this woman is not pregnant, would one pay heed to him and would we allow the yavam to consummate levirate marriage?
Certainly not, as is evident from the fact that a minor girl, who cannot become pregnant, must also wait three months. Evidently, the requirement to wait three months applies in all situations, even those in which the reason for the requirement is irrelevant. Therefore, by virtue of that decree the yevama is considered to be fundamentally ineligible for levirate marriage and similarly is ineligible for ḥalitza.
We explore the use of Elijah in determining halacha or in this case acting as an ultrasound to detect a pregnancy in the talmud and other historical aspects of Eliyahu especially how he crept into the haggadah.