Halakhah su Levitico 18:9

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.

To not reveal the nakedness of his sister, from any angle that she is his sister: To not reveal the nakedness of his sister - whether she is a sister only from the father or whether she is a sister only from the mother; whether she is a sister from a woman raped by his father, or whether she is a sister from a mother from infidelity. From all of these angles, she is called his sister, and [he] is liable for her, as it is stated (Leviticus 18:9), "The nakedness of your sister - the daughter of your father or the daughter of your mother, whether born into the household or born outside." And if you shall say, "If so, what is this that is stated in the Torah (Leviticus 18:11), 'The nakedness of the daughter of the wife of your father, who has born into your father’s household'; and what was the need for this verse, is he not liable for her on account of [her being] his sister" - this verse was truthfully only stated to make the daughter of the wife of the father when she is his sister, a sexual prohibition on its own, and as we will write below in its place (below, Sefer HaChinukh 196). And it comes out that one who has intercourse with his sister, who is the daughter from the marriage of his father, is liable for two: on account of the nakedness of his sister and the nakedness of the daughter of the wife of his father. (Yevamot 22b). And if he has intercourse with her inadvertently, he is liable for two sin-offerings.

To not uncover the nakedness of his sister from the father, and she is the daughter of the wife of the father: To not uncover the nakedness of the daughter of the wife of the father when she is his sister - meaning to say, that she is the daughter of his father. As if she was the daughter of the wife of his father that is not his sister, we have already said above (Sefer HaChinukh 192) that he is permitted to marry her from the outset. Rather, the understanding [of the prohibition] is that it is the daughter of the wife of his father and she is his sister, as it is stated (Leviticus 18:11), "The nakedness of the daughter of your father [...] you shall not reveal." And I have already written above regarding the nakedness of the sister that this negative commandment comes in order to make his sister from the wife of his father a sexual prohibition by itself - and similar to what I wrote about his mother (Sefer HaChinukh 190): that if she is the wife of the father, he is liable with her for two - on account of the mother and on account of the wife of the father. And the proof is from that which they, may their memory be blessed, said in Yevamot 22b, "The Sages learned, 'He who has intercourse with his sister who is the daughter of his father’s wife, is liable on account of his sister and on account of the daughter of his father’s wife.' Rabbi Yose beRebbi Yehudah says, 'He is liable only on account of his sister alone.' What is the reason for the opinion of the rabbis? They said, 'How is it? Since it is written in the Torah, "The nakedness of your sister, the daughter of your father, etc." (Leviticus 18:9), for what do I need, "The nakedness of the daughter of the wife of your father, who has born into your father’s household," (Leviticus 18:11)? We understand from this, [that] it is to make him liable on account of his sister and on account of the daughter of his father’s wife.'" It comes out that there is only one negative commandment with one who lays with his sister who is from a woman that his father raped; but one who lays with his sister from a woman that his father married is liable for [the transgression of] two negative commandments - and [so] if inadvertent, he is obligated to bring two sin-offerings. And 'the judgments of God are righteous': Since the Torah distanced the sister, it is fitting to be stringent with the one that is his full sister - meaning to say from marriage, as she is called the [true] sister (of her mother). And like this matter did Ramban, may his memory be blessed, write (Ramban on Leviticus 18:9).