Tractate Yevamot - Chapter 15 - Mishnah 10 יבמות פרק ט״ו משנה י׳

[If a woman states] “a brother-in-law was given to me [while I was] in a country beyond the sea”, and afterwards she states, “my husband died and afterwards my brother-in-law died” or “my brother-in-law died and afterwards my husband died”, she is believed. If a woman and her husband and her brother-in-law went to a country beyond the sea, and she [on returning home] stated, “my husband died and afterwards my brother-in-law [died]” or “my brother-in-law [died] and afterwards my husband [died]” she is not believed; For a woman is not to be believed when she asserts “my brother-in-law is dead”, in order that she may marry again; Nor [is she believed when she states that] her sister is dead, in order that she may enter his house. A man also is not believed when he states “my brother is dead”, so that he may have yibbum with his wife, nor [when he states that] his wife is dead, in order that he may marry her sister.