Mishnayos Yevamos Perek 15 Mishnah 10

If she said: A yavam was born for me overseas, i.e., when the family left the country her husband did not have a brother, and she claims that in the meantime a brother was born to her husband, and she also said: My husband died and afterward my yavam died, or: My yavam died and afterward my husband died, in either case she is deemed credible. This is because when she left she was not presumed to require levirate marriage, and the suggestion that her husband now has a brother is based solely on her testimony. However, if she went, she and her husband and her yavam, overseas, and upon her return she said: My husband died and afterward my yavam died, or: My yavam died and afterward my husband died, she is not deemed credible, as a woman is not deemed credible if she says: My yavam died, in order that she may marry another man. And she is not deemed credible if she says that her sister died, in order that she may enter the house of her sister’s husband. And a man is not deemed credible if he says: My brother died, so that he may enter into levirate marriage with his brother’s wife, and he is not deemed credible when he says that his wife died, in order that he may marry his wife’s sister. The Sages accepted impaired testimony of this kind only when there was a concern about creating a situation of a deserted wife.

נִתַּן לִי יָבָם בִּמְדִינַת הַיָּם, אָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת יְבָמִי, יְבָמִי וְאַחַר כָּךְ בַּעְלִי, נֶאֱמֶנֶת. הָלְכָה הִיא וּבַעְלָהּ וִיבָמָהּ לִמְדִינַת הַיָּם, אָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת יְבָמִי, יְבָמִי וְאַחַר כָּךְ בַּעְלִי, אֵינָהּ נֶאֱמֶנֶת. שֶׁאֵין הָאִשָּׁה נֶאֱמֶנֶת לוֹמַר מֵת יְבָמִי, שֶׁתִּנָּשֵׂא. וְלֹא, מֵתָה אֲחוֹתִי, שֶׁתִּכָּנֵס לְבֵיתוֹ. וְאֵין הָאִישׁ נֶאֱמָן לוֹמַר מֵת אָחִי, שֶׁיְּיַבֵּם אִשְׁתּוֹ. וְלֹא, מֵתָה אִשְׁתִּי, שֶׁיִּשָּׂא אֲחוֹתָהּ: