Why isn’t she believed when she says my yavam has died? TB Yevamot 118

We previously learned that a woman’s testimony of her husband died is excepted even though normally Jewish law requires two male witnesses for a fact to be established. The Gemara brings a variation of a theme that it has been discussing. “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.” (Srfaia.org translation) if she is believed concerning her husband, why isn’t she believed concerning her yavam? There are two reasons may be suggested why she is believed when she said her yavam has died. Perhaps she hates her brother-in-law and doesn’t want to marry him; consequently, she will not do her due diligence to find out whether he has truly died. Rambam suggests a different solution. The woman’s believed when comes to testify about the death of her husband because a married woman (אשת איש) having an affair with another man carries a severe penalty. Consequently, she would make sure that he is indeed dead. But not marrying her yavam only violates a negative prohibition whose penalty is not as severe. She may not be as careful observing the laws of levirate marriage.