Yevamot 10 - Case number sixteen
Even though Rabbi Yehudah sharply rebuked Levy for his suggestion , the latter insisted on his understanding and added case number sixteen to the list of women who exempt their co-wives from yibum. Here it is.
Let us say one's mother was legally married to his father (a normal case), but after his father's death she "married" his brother (from another woman). The "marriage" of his mother and his brother never took legal effect, because she was previously married to the brother's father. Therefore, if his brother now dies, there isn't even a suggestion that he should marry his mother in yibum.
However, if his father "violated" his mother - that is, lived with her without getting married - then his brother (the son of his father from another woman) can indeed marry his mother. If this brother now dies, our protagonist finds himself in the position of a yibum; but since he cannot marry his mother, she does not get a yibum or chalitzah from him, and his brother's other wives also don't need a yibum or chalitzah. This is exactly the situation of the fifteen cases above , and this is the case number sixteen that Levy wanted to include.
If so, why did Rabbi Yehudah disagree? - Because this scenario involves an "if-he-does" case, a prohibited act of son "marrying" his father's former wife, and Rabbi Yehudah claims that such cases which involve a transgression were not included in the basic rule .
Art: Double portrait of a father and son by Jacob Gerritsz. Cuyp