Now, if the niddah impediment and the onen impediment prevent the union at the time of the death, then the yibum union should remain permanently prohibited, consistent with the “once prohibited, always prohibited” rule. Tosafot, in addressing the niddah question, differentiates between the ervah prohibition on the one hand and the niddah prohibition on the other. True, concedes Tosafot, both the niddah prohibition and the ervah prohibition in the car crash situation described above are temporary prohibitions. But the ervah prohibition against Shimon marrying his wife’s sister during her lifetime is unique to Shimon. The prohibition against relations with a niddah applies to all. Consequently, the “once prohibited always prohibited” rule does not apply to the niddah situation.
Indeed, this answer can also be applied to the onen situation. Other commentators (Maharitz Chayut, Yevamot 2) differentiate between a temporary prohibition whose duration is predictable, and a temporary prohibition of unpredictable duration. The “once prohibited always prohibited” rule applies to a temporary prohibition of unpredictable duration, such as the car crash case. It does not apply to the temporary prohibition of predictable duration, such as the niddah or onen case.
Raphael Grunfeld’s new book, “Ner Eyal on Seder Nashim, Nezikin, Kodashim, Taharot and Zera’im,” will be published shortly.