176 Yevamot 15/10 & 16/1

(10) [If] a woman [states], "I was given a Yavam [one upon whom has fallen the obligation to perform Levirate marriage] overseas" [that is, her mother-in-law had a son there], and then states, "My husband died and then later my Yavam died", [or] "My Yavam died and then later my husband died," she is trusted. [If] she and her husband and her Yavam went overseas, [and] she states, "My husband died and then later my Yavam died", [or] "My Yavam died and then later my husband died," she is not trusted, since a woman is not trusted to state "My Yavam died" [which would allow her] to marry, and [she is] not [trusted to state] "My sister died" [which would allow her] to enter [her sister's husband's] house. And a man is not trusted to state, "My brother died" [which would allow] him to perform Yibum for his wife, and [he is] not [trusted to state] "My wife died" [which would allow] him to marry her sister.

(1) [If] a woman's husband and her rival went overseas and they [witnesses] came and told her, "Your husband died", she may not marry, and may not undergo Yibum [Levirate marriage wherein a man weds his childless brother's widow] until she knows whether her rival is pregnant. If she has a mother-in-law she need not worry [that the mother-in-law has had a son]. [If] she [the mother-in-law] left pregnant, she need worry [that she has a Yavam]. Rabbi Yehoshua says: She need not worry.