Halakhah zu Bereschit 20:12

However, the Tur (Peirush Tur Ha’aroch on Bereishit 46:10) explains this midrash in a manner that seemingly indicates the exact opposite, that the ovum donor is the halachic mother in a case of surrogate motherhood. In analyzing the midrash (quoted by Rashi on Bereishit 46:10) that Shimon married his sister, Dinah, the Tur wonders why their union did not constitute incest. After all, Shimon and Dinah were both children of Leah, and marrying a maternal sister was prohibited even before the giving of the Torah. The Tur answers that, as quoted above from Targum Yonatan, Dinah began in Rachel’s womb. Even after she was switched to Leah’s womb, the Halachah still considered her to be Rachel’s daughter, so she and Shimon thus had different mothers. Before the Torah was given, one was allowed to marry a paternal half-sister.¹¹Even nowadays, Noachide Law (Halachah pertaining to non-Jews) permits marrying a paternal half-sister, while a Jew may not marry any half-sister; see Vayikra 18:9, Rambam (Hilchot Melachim 9:5), and Rashi (Bereishit 20:12). Therefore, Leah’s son, Shimon, did not violate the Halachah when he married Rachel’s daughter, Dinah. We thus see that according to the Tur, the Halachah defines motherhood by the woman whose egg forms the fetus, even if another woman gives birth to the baby. Of course, Aggadic passages usually cannot serve as definitive halachic proofs.¹²See Yerushalmi (Pe’ah 2:4), Encyclopedia Talmudit (1:62), Teshuvot Yabia Omer (vol. 8, Even Ha’ezer 21:2), and Nishmat Avraham (3:17). Nevertheless, the Tur’s words merit serious halachic consideration, especially because he is explaining how to understand the story from a halachic perspective.