According to the laws of the Torah, a cohen is forbidden to marry a harlot or a defiled woman, for it is written “He shall not take a harlot or a defiled woman” (Leviticus 21:7). According to the sages a woman is considered a harlot if she had sexual relations in contradiction to the laws of the Torah. A defiled woman, the sages ruled, is one who had sexual relations which would be forbidden to a cohen. One sage, Rav Papa, asked his colleague: If a Jew has sexual relations with his sister, his sister is considered a harlot according to the widely-accepted law. Is the sister also considered a defiled woman? What are the issues under debate? On one hand, they ruled that a woman is considered defiled only if she is involved in a sexual relationship prohibited to a cohen, and a sister is not forbidden because one is a cohen; every man is forbidden to have sex with his own sister. On the other hand, since sex with one’s sister is a very serious transgression, perhaps she should be considered defiled, even though the prohibition has nothing to do with the priesthood.
Answer: The sister does not become a defiled woman, only a harlot. Therefore another sage, Rav Ashi, concludes that if a cohen has sex with his sister, the sister is considered a harlot. If he has sex with her again, she is also considered a defiled woman, for she was judged to be a harlot and a cohen may not have a sexual relationship with a harlot.
(Babylonian Talmud, Tractate Kiddushin 77b)