Related for Kiddushin 151:5
אמר רב נחמן אמר רבה בר אבוה ממזר מאחותו וממזר מאשת אח נתערבו בהן מאי קא משמע לן יש ממזר מחייבי כריתות ניתני חדא מעשה שהיה כך היה
But here [in respect to marriage], wherein are they not well-versed? - Because they are not well-versed in the law of betrothal and divorce.<span class="x" onmousemove="('comment',' Thus, a woman may have been validly betrothed, yet they thought it invalid and permitted her to marry another, the issue by whom is mamzer. Another Cuthean, however, may be quite legitimate; therefore R. Eleazar forbids Cutheans to marry each other.');"><sup>9</sup></span> R'Nahman said in Rabbah B'Abbuha's name: A mamzer by a sister and a mamzer by a brother's wife became mixed up among them [the Cutheans].<span class="x" onmousemove="('comment',' Therefore one Cuthean may not marry another.');"><sup>10</sup></span> What does he inform us? - That there is mamzer from those who are liable to kareth.<span class="x" onmousemove="('comment',' By specifying a mamzer from an incestuous union with a sister, his intention is to teach that the issue of such, though forbidden only on pain of kareth, is mamzer, in opposition to the view (Yeb. 49a) that only when the union involves death by the court is the issue mamzer (v. Rashi) .');"><sup>11</sup></span>