The 333rd prohibition is that one is forbidden from having relations with the daughter of one's father's wife, if she is also a sister.1
The source of this commandment is G‑d's statement2 (exalted be He), "Do not commit incest with a daughter that your father's wife has borne to your father. She is your sister."
This prohibition comes to single out the daughter of your father's wife as a separate incestual prohibition. The proof that the daughter of your father's wife is a distinct prohibition is from the following law: if a man has relations with his sister, and her mother is married to his father,3 he has transgressed two prohibitions — incest with his sister and with the daughter of his father's wife. This is like one who has relations with his mother, who has transgressed two prohibitions — incest with his mother and with his father's wife, as explained above.4
This is how our Sages expressed this idea in the second chapter of Yevamos:5 "Our Sages taught, 'One who has relations with his sister, and she is also the daughter of his father's wife is culpable for committing incest with one's sister and for committing incest with the daughter of one's father's wife.' Rabbi Yossi the son of Rabbi Yehuda said, 'He is only culpable for the prohibition of incest with one's sister.' What is the reasoning of the Sages? They say, let us examine [the verses]: it is already written,6 '[Do not commit] incest with your sister, your father's daughter.' Why do we need the verse,7 '[Do not commit] incest with a daughter that your father's wife has borne to your father'? This verse makes him guilty on two counts: incest with one's sister and with the daughter of one's father's wife."
The punishment for violating this prohibition — i.e., incest with one's sister, the daughter of one's father's wife — is solely8 kares9 if done intentionally. If the act was unintentional, he must bring a sin-offering.10
