Why does the Torah see fit to add another Pasuk concerning a sister?
Ramban #2 (citing Yevamos, 22b): To teach us that someone who transgresses be'Shogeg, 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. 5 ).
And who does not therefore fall under the category of "bas Eishes Avichah". Refer also to 18:9:2:2*.
Bearing in mind the distinction that the Pasuk made earlier with regard to his mother's daughter (Refer to 18:9:2:3).
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).
And is therefore Chayav to bring two Chata'os.