Yevamos 15:10-16:1

Yevamos 15:10

If a woman returned from overseas and reported that a brother-in-law was born there, then her husband died, followed by the brother-in-law, or that the brother-in-law died, followed by the husband – in either case she is believed (because she retains her status quo). If a woman went overseas with her husband and brother-in-law, then she returns and reports that her husband died, followed by the brother-in-law, or the brother-in-law died, followed by the husband – in either case she is not believed. A woman is not believed if she reports that her brother-in-law died, as this might just be so that she can avoid yibum, or if she reports that her sister died, as this might just be so that she can marry her sister’s husband. A man is not believed if he reports that his brother died, as this might just be so that he can perform yibum with his sister-in-law, or if he reports that his wife died, as this might just be so that he can marry her sister.

Yevamos 16:1

If a woman’s husband and her co-wife went overseas, then she received word that her husband died, she may not marry or perform yibum until she finds out whether or not the other wife is pregnant (because if she is, yibum may no longer be necessary). If the husband went overseas with his mother and dies, we need not be concerned that the woman’s mother-in-law gave birth to a brother-in-law unless she was pregnant when they left. Rabbi Yehoshua says that even then one need not be concerned (because of a double doubt: did she give birth or miscarry? And if she gave birth, was it a boy or a girl?).