The Torah in Parashat Ahareh-Mot (Vayikra 18:14) commands regarding one’s father’s brother, “El Ishto Lo Tikrab, Dodatecha Hi” – “Do not approach his wife; she is your aunt.” This verse establishes a prohibition against having relations with the wife of one’s father’s brother. This prohibition applies even after the uncle died or divorced his wife; once one’s father’s brother marries a woman, the nephew may never have an intimate relationship with that woman. Furthermore, this prohibition applies even to a woman whom the uncle merely betrothed, even before the marriage was completed.
The Biblical command applies only to the wife of one’s father’s brother from the same father. If one’s father has a half-brother from the same mother, but not from the same father, that the nephew is, on the level of Torah law, allowed to marry a woman to whom the uncle had been married. The Sages, however, enacted a provision prohibiting such a relationship.
This prohibition applies in all times and in all places. If one intentionally transgressed this command, and had a relationship with his father’s brother’s wife while she was married to the uncle, then the nephew and the aunt are punished by Henek (execution by strangulation), for violating the prohibition of adultery. If the uncle and aunt were no longer married, then their forbidden act is punishable by Karet. If witnesses saw the act, then the violator is liable to Malkut. If the act was committed unintentionally, then the violator must bring a Hatat (sin-offering). If one had intercourse with the wife of his father’s brother from the same mother but not from the same father, then he receives Malkut by force of Rabbinic enactment.
The Torah later (Vayikra 20:20) says about those who engage in such a relationship, “Aririm Yamutu” – “they shall die childless.” The Gemara (Yebamot 55) explains this to mean that the violators will be unable to beget children, and if they already had children, the children will die during their lifetime. Rashi writes that this verse establishes that part of the punishment of Karet is childlessness; the Torah here states that whenever someone deserves Karet, his punishment will include having no offspring. Tosafot, however, disagree, and maintain that only when one is guilty of a sin involving Arayot (forbidden intimate relationship), his punishment includes childlessness. Sinners who are liable to Karet for other offenses, by contrast, are not, in Tosafot’s view, subject to this punishment of childlessness.
During the Vidui (confessional) prayer on Yom Kippur, we confess “Al Hataim She’anahnu Hayabim Alehem Karet Va’ariri” – “for sins for which we are liable to Karet and childlessness.” This text is readily understandable according to Rashi’s opinion, that every sin which carries a punishment of Karet also carries the punishment of childlessness. Naturally, then, we mention these two punishments together, in reference to the same category of sin. According to Tosafot’s opinion, we must explain this prayer text as referring first to all sins punishable by Karet, and then specifically to those that carry the additional punishment of childlessness – meaning, sins of Arayot. Interestingly, the Maharil (Rav Yaakob Moelin, Germany, 1365-1427) had the practice to include the word “Va’ariri” in this passage only during the Minha service on Yom Kippur afternoon. The likely explanation is that he followed Tosafot’s opinion, that the punishment of “Ariri” is reserved only for violations of Arayot. The Maharil therefore made mention of this punishment specifically during Minha, when the portion read from the Torah is the section in Parashat Ahareh-Mot which introduces the Arayot prohibitions. Having just read the section of Arayot, the Maharil felt, we should make special mention of these sins in our confessional prayer.