QUESTION: The Mishnah states that neither a mother-in-law nor a daughter-in-law may testify about the death of the other's husband. Each woman is suspected of maliciously attempting to harm the other by lying about the other's husband, due to the enmity they have for each other. The Gemara explains that the mother-in-law despises her daughter-in-law because her daughter-in-law "consumes all of the fruits of her labor" (because her son gives his wife everything his mother prepared for him).
The Tana'im disagree about the basis for the converse hatred: why does the daughter-in-law despise her mother-in-law? Rebbi Yehudah maintains that she despises her mother-in-law because her mother-in-law reveals to the daughter-in-law's husband all of the secrets about what she does in private. The Rabanan maintain that she despises her mother-in-law simply because her mother-in-law despises her, as the verse says, "As water reflects a face back to a face, so one's heart reflects the other's heart" (Mishlei 27:19). The Mishnah follows the view of the Rabanan, whose view is the Halachah.
The Gemara then discusses the trustworthiness of a woman who testifies about the husband of her potential mother-in-law ("Chamosah ha'Ba'ah l'Achar mi'Kan"). This refers to a woman who is married but will fall to Yibum if her husband dies childless, and her husband's brother has a different mother than her husband. The Gemara asks whether such a woman hates her potential mother-in-law or not.
The Gemara cites proof from the Mishnah later (118a) which discusses a case in which a woman's husband and her father-in-law traveled abroad. The woman testifies that her husband, and the husband of her mother-in-law, died. The Mishnah states that she is not believed to permit her mother-in-law to remarry. The Gemara asserts from here that an anticipated hatred of a mother-in-law is cause for concern, even though the hatred presently does not exist.
RASHI explains that the Gemara's proof is that the Mishnah teaches that the daughter-in-law hates her mother-in-law even when both husbands are in Medinas ha'Yam and the mother-in-law does not have the opportunity to reveal to her son the secrets of her daughter-in-law. Although the daughter-in-law has no reason to be angry at her mother-in-law at the present moment since her husband is away, nevertheless she expects that when her husband returns her mother-in-law will reveal to him her secrets.
Rashi mentions only Rebbi Yehudah's reason for why a daughter-in-law hates her mother-in-law. Why does he not explain the Gemara's proof according to the reason of the Rabanan, who explain that the daughter-in-law hates her mother-in-law simply because her mother-in-law hates her (reciprocal hatred)?
ANSWER: Rashi understands that according to the Rabanan, the Gemara has no proof from the Mishnah later. The reason the mother-in-law hates the daughter-in-law is that the daughter-in-law eats the fruit of her labor. Consequently, according to the Rabanan, as long as the daughter-in-law benefits from the work of the mother-in-law she hates the mother-in-law because of reciprocal hatred. Hence, she hates the mother-in-law even when their husbands are away, because the mother-in-law still hates her at that moment.
Accordingly, the Gemara's proof from the Mishnah later is only according to the reasoning of Rebbi Yehudah, who maintains that the daughter-in-law hates the mother-in-law because she reveals her secrets. According to that reason, the Gemara has a valid proof that Beis Din must be concerned for anticipated hatred. She has no reason to hate her mother-in-law at the present moment, while her husband is away (and her mother-in-law cannot tell him any secrets).
The RITVA seems to understand Rashi in this way as well. The Ritva adds that if the Gemara is able to prove only according to Rebbi Yehudah that the daughter-in-law lies in Beis Din because of anticipated hatred for her mother-in-law, what proof is there that the Rabanan agree with him on this point? The Ritva answers that there is no reason to assume that the Rabanan should not agree with Rebbi Yehudah on that point. Hence, if Rebbi Yehudah maintains that a woman testifies falsely because of anticipated hatred for her mother-in-law, it may be assumed that the Rabanan agree.