Before us is Mishnah 1 of chapter 11 of Maseches Yevamos, which deals with a woman who was raped or seduced and the implications of this for the prohibitions against marrying one's relatives.
"Nos'in al ha'anusah v'al hamefutah":
A man who raped or seduced a woman, that is, had relations with her outside the framework of marriage, is permitted afterward to marry her relatives: her daughter, her mother or her sister. The reason is straightforward: these prohibitions of relatives take effect only by virtue of marriage. A man married to a woman is forbidden to her relatives, but one who merely had relations with her and is not married to her is not forbidden to her relatives.
In the reverse case, however, the ruling is reversed: "v'ha'ones v'hamefateh al hanesuah - chayav" - one who rapes or seduces a woman who is a relative of the wife he is married to, her mother, her sister or her daughter, is liable for the punishment set for those prohibited relations.
The woman his father raped and the woman his son raped:
The Mishnah continues and expands on this matter: "Nosei adam anusas aviv umefutas aviv, anusas b'no umefutas b'no" - two rulings are included here:
The woman his father raped or seduced - a man is permitted to marry a woman with whom his father had relations outside of marriage, whether by force or by seduction. The Torah forbade only his father's wife, meaning a woman married to his father through kiddushin, and not a woman with whom he had relations outside the framework of marriage.
The woman his son raped or seduced - similarly, a man is permitted to marry a woman whom his son raped or seduced, since the Torah forbade to him his daughter-in-law, who was married to his son.
The opinion of Rabbi Yehudah:
Regarding the woman his father raped, Rabbi Yehudah disagrees and forbids it: "Rabbi Yehudah oser ba'anusas aviv umefutas aviv" - in his view, a man may not marry a woman whom his father raped or seduced.
In an earlier section, at the beginning of the tractate, we noted an implication that follows from this opinion: a man's mother can never come before him for yibbum. For the woman who is his mother could never be married to his brother under any circumstances, even if that brother was born from relations outside of marriage. But if a man were permitted to marry a woman who had relations with his father, one could imagine a situation in which a man would marry the woman who is his brother's mother.
In summary: in this Mishnah we learned that the prohibitions of relatives stem from marriage alone: one who had relations with a woman by force or by seduction is permitted to her relatives, while one who rapes or seduces a relative of his married wife is liable. So too, a man is permitted to the woman his father raped or seduced, and to the woman his son raped or seduced, while Rabbi Yehudah forbids the woman his father raped or seduced, an opinion from which it follows that a man's mother never comes before him for yibbum.