Commentary for Sotah 6:2

אני ראיתיה שנטמאת – in that retirement [of hers] under suspicious circumstances which is through two witnesses according to Rabbi Yehoshua or through the flying bird according to Rabbi Eliezer.

If one witness said, “I saw that she was defiled”, she does not drink the water. Not only that, but even a slave, male or female, is believed even to disqualify her from receiving her ketubah.
Her mother-in-law, her mother-in-law’s daughter, her rival wife, her sister-in-law, and the daughter of her husband are believed, not to disqualify her from receiving her ketubah, but that she should not drink.
Above in mishnah 1:3 we learned that if witnesses testify that the woman had committed adultery with the suspected man (and not just that they saw her secluded with him) she doesn’t drink the sotah waters. The waters test only those with regard to whom there is doubt. Today’s mishnah teaches that if even one witness saw her committing adultery, she does not drink the sotah waters and she loses her ketubah.
Section one: Generally in Jewish law two witnesses are needed for a court to act. However, in this case since there has already been a process of warning the woman, even one witness is sufficient to prove that she has committed adultery. If this happens she does not drink the sotah waters, as would a suspected adulteress. Furthermore, even a witness who is normally not allowed to testify may testify in this case. This includes slaves, both male and female. Such testimony is sufficient even to disqualify her from receiving her ketubah. This is a more significant step because it causes her to lose money promised to her.
However, there is a small list of people who are suspected of lying under such circumstances, and therefore while they can testify against her and thereby prohibit her to her husband, they cannot cause her to lose her ketubah. We have seen this list before in Yevamoth 15:4. There we learned that these women cannot testify that a woman’s husband is dead. The fear is that they are lying because they have a “natural” hatred for the woman. The suspicion that they hate this woman is what disqualifies them from testifying here as well, at least with regard to her ketubah. However, with regard to causing her to become prohibited to her husband, they are believed.

לא היתה שותה – for one witness is believed concerning her even to cause her to lose her Ketubah settlement for there is a basis for it.

אף לפסלה מכתובתה – that she should not drink [the bitter waters] not take [the settlement of] her Ketubah.

יבמתה – the wife of her husband’s brother.

ובת בעלה – from another wife.

הרי אלו נאמנות – for this testimony, even though they are ineligible for all other testimony which is to her detriment because the Torah believed all testimony that there is concerning her. But, however, not to make her ineligible for her Ketubah [settlement], for they hate her, but rather, that she should not be suspected by her husband of having been unfaithful.