It all depends on what the clans involved define as incest.
I can see a clan where the woman comes into the clan as part of the clan's family considering a brother's wife as kin.
However, a clan where the men join the clan when they marry a woman from the clan might think that the husband's brother does not belong to the clan, so it is not incest. In fact, they might think that the brother is the best person to marry the woman, as it keeps family ties as they were before the death of the brother.
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