1) Robert, it seems to me that you understand that a full-sister is a combination of your father's daughter and your mother's daughter. Therefore, you understand that there is a prohibiiton on the father's daughter, and a separate prohibition on the mother's daughter, and it follows that if a man has relations with his full sister he transgresses two separate prohibitions.

However, I do not think that this is the correct way of looking at it. Rather, there is one prohibition against one's full sister. This is the chief prohibition. In addition, the Torah says that even if she is not one's full sister but only a half-sister, she is also forbidden. It does not mean that a full-sister is a double half-sister.

According to this, it is not possible that one verse could give us two prohibitions against one sister.

We find support, bs'd, for this from the Gemara in Makos 14a, cited by Rashi here (at the top of 3a). The Gemara says that even though the verse (Vayikra 18:9) tells us that one may not marry the daughter of his father or the daughter of his mother, had the Torah not commanded explicitly that one may not marry the sister, we would not have known the latter prohibition. This suggests that the three prohibitons are independent, so there is no reason to think that one is liable for two sets of Malkus for marrying one's full sister.

2) However, the Gemara in Yevamos 22b seems to say something similar to your argument, Robert.

a) The Gemara there is based on the verse, "You shall not reveal the Ervah of the daughter of your father's wife" (Vayikra 11:11). The Gemara (Yevamos 22b) cites a Beraisa in which the Chachamim say that one who has relations with his sister, the daughter of his father's wife, is liable for both of these prohibitions.

b) This seems to be similar to your understanding that it is possible to commit two transgressions with just the one sister.

c) However, in reality, I think there may be a difference between the prohibition on a woman who is one's sister from both the father's side and the mother's side, and the sister who is the daugher of one's father's wife.

d) This is because the prohibition against the daughter of one's fathers' wife may not be because she is one's sister, but rather this prohibition is a sort of extension of the prohibition against marrying one's father's wife. Not only may one not marry one's father's wife but one may not even marry the daughter of one's father's wife.

e) This is not similar to the prohibition against marrying one's sister from one's mother. The latter prohibition is not an extension of the prohibition against marrying one's mother.

Thank you for your very interesting idea.

Kol Tuv,

Dovid Bloom

1)I have already given different answers to this question, but I have now found, bs'd, that Rashi Sanhedrin 53a in the Mishnah DH Haba Al Ha'Eim writes that for biyah with the sister one is liable for 2 prohibitions; one for the sister from a mutual father, and one for the sister from a mutual mother. So this, on the surface of it, is a support for your understanding, Robert, that there couild be 2 prohibitions; one for your father's daughter, and one for your mother's daughter.

2) The Rashash, on the above Rashi, writes that an amendmnent must be made. It should read "to oblige him because of his sister; who is his father's sister and his mother's sister. This is in fact the text that we have in Makos 14a and it is also the text of the Nimukei Yosef in Sanhedrin 53a.

3) However the Binyan Shlomo, cited by Metivta in Biurei Rashi #86, takes Rashi at face value; according to the text we have in Rashi. Someone who has biyah with his sister where they are brothers and sisters from both parents, is liable twice; once for the sister from the father, and once for the sister from the mother.

4) According to this, the answer to your question is that you do indeed get 2 prohibitions; one for your father's daughter and one for your mother's daughter.

Dovid Bloom

1) I have already given different answers to this question, but I have now found, bs'd, that Rashi in Sanhedrin 53a (in the Mishnah, DH ha'Ba Al Ha'Em) writes that for Bi'ah with the sister, one is liable for two prohibitions -- one for the sister from a mutual father, and one for the sister from a mutual mother. So on the surface of it, this is a support for your understanding, Robert, that there could be two prohibitions, one for his father's daughter and one for his mother's daughter.

2) The Rashash, on the above Rashi, writes that an amendmnent must be made. It should read "to obligate him because of his sister who is his father's sister and his mother's sister." This is in fact the text that we have in Makos 14a and it is also the text of the Nimukei Yosef in Sanhedrin 53a.

3) However, the Binyan Shlomo (cited by the Mesivta edition in Bi'urei Rashi #86) takes Rashi at face value, according to the text we have in Rashi here. Someone who has Bi'ah with his sister from both parents is liable twice, once for the sister from the father, and once for the sister from the mother.

4) According to this, the answer to your question is that he indeed gets two prohibitions, one for his father's daughter and one for his mother's daughter.

Dovid Bloom