Why does the Torah add another Pasuk forbidding a sister?
Rashi: To preclude the daughter of one's father from a Shifchah Cana'anis and from a Nochris from the Din of "Moledes Chutz" 1 - since she is not subject to Kidushin [with any man].
Ramban #1: To extrapolate that here she is his father's daughter through marriage - but "Moledes Chutz" - in Pasuk 9 2 - is not through marriage. 3
Ramban #2 (citing Yevamos, 22b): To teach us that someone who transgresses, and has relations with his sister who is also the daughter of his father's wife who is not his mother, transgresses two La'avin. 4
Moshav Zekenim: The earlier Pasuk discussed a father's daughter not through marriage, the current Pasuk, a father's daughter through marriage. 5
Makos 5b: We already learned (in Pasuk 9) about his father's daughter not through marriage. Since the Torah needed to teach us about his father's daughter through marriage, we can extrapolate that one cannot punish 6 via a Kal va'Chomer.
Who does not therefore fall under the category of "Bas Eishes Avicha". Refer also to 18:9:2:1 & 18:9:2:2*.
Despite the fact that one is not sure that it is his father's daughter, and despite the fact that a Nochri is permitted to marry his paternal sister (Ramban).
If he was Meizid he gets two sets of lashes; if he was Shogeg, he brings two Chata'os. (PF)
Moshav Zekenim: Even though we can learn it from a Kal va'Chomer. Refer to 18:10:1:1*.
See Torah Temimah, note 31.
