Yevamot 97 - Riddles
If one's father violated or seduced a woman (or otherwise had an affair with her), then the son can still marry her. The prohibition to marry the wife of one's father does not apply here. Rabbi Yehudah forbids the son to marry such a woman. The verse itself is talking only about a legal wife, but Rabbi Yehudah derives his additional prohibition from the context. Even the first teacher will agree that the Sages later prohibited such marriage.
Now a few riddles. Who can say about a man that he is "my paternal brother but not a maternal one, he is my mother's husband, and I am his wife's daughter?" This can happen legally following the first teacher, but not Rabbi Yehudah, as follows. A man (Jacob) violated a woman and had a daughter with her. Reuven (Jacob's son from another woman), married the woman whom his father violated. Reuven's daughter can say that Reuven, her paternal brother, married her mother.
Another one. A woman says: he is my brother, and he is my son. I am the sister of this child of mine whom I carry on my shoulders. Answer: a man had relations with his daughter, and fathered a son by her. The child is at once her son and her paternal brother.
Yet another one: a woman says, "Peace to you, my son; I am your sister's daughter." Answer: one had relations with his daughter's daughter and fathered a son. She is at once the child's mother and its paternal sister's daughter.
Why do we need such riddles? Some say - to sharpen the minds of the Torah scholars, while others - that Queen Sheba posed these riddles to King Solomon, to test his wisdom.
Art: Sisters In The Sewing Room by Fritz von Uhde