Yevamot 117B

תא שמע, אמרה: ״מת בעלי ואחר כך מת חמי״⁠ — תנשא ותטול כתובה, וחמותה אסורה. מאי טעמא חמותה אסורה? לאו משום דאמרינן לא בעלה מיית ולא חמוה מיית, והא דקאמרה הכי ⁠— לקלקולא לחמותה הוא דקמיכוונא,

Come and hear a resolution to this dilemma. If she said: My husband died and afterward my father-in-law died, she may marry and take her money from the marriage contract, and her mother-in-law is prohibited to remarry; she is not deemed credible to testify for her mother-in-law, as already stated. The Talmud clarifies: What is the reason that her mother-in-law is prohibited to remarry? Is it not because we say: Perhaps her husband did not really die, and she is still her mother-in-law, and her father-in-law did not die either, and the reason that she says this statement is that she intends to ruin her mother-in-law?

סברה: לבתר שעתא לא תיתי (תצטערן).

The Talmud elaborates. She reasons: Later, when the husbands arrive, she will not come back and trouble me, because if the mother-in-law relies on this testimony and remarries she will no longer be able to return to her original husband, and she will be out of her daughter-in-law’s life. This shows that there is a concern that a daughter-in-law might lie in order to prevent future family relationships from coming into being. Similarly, a woman should be suspected of lying with regard to her future daughter-in-law.

דלמא שאני התם, דרגיש לה צערא.

The Talmud rejects this suggestion. Perhaps it is different there, as the daughter-in-law has already felt oppressed by her mother-in-law. In other words, she is suspected of lying because she had previous dealings with that woman, whereas in the case of a future mother-in-law, with whom she had no previous dealings, there is no such concern. Consequently, the dilemma cannot be resolved from this case.