daf bit: Yevamot 26
However, if one of the sisters is forbidden to one of the (remaining) brothers under the laws of incest and, similarly, the other sister is forbidden to the other brother, then the non-forbidden pairs can in fact marry because there is no rivalry -- because each man could only marry one of them (and a man is not allowed to marry two sisters) there is no competition between the two sisters to see who will marry which man. (26a)
(I am not quite sure how the laws of incest mentioned here would come into play. This might involve half- and step-siblings, maybe.)
Somebody asked me this morning about the word "levirate". It's another of those unhelpful translations -- the Hebrew word for this is yibum (hence tractate "yevamot"), or in English (via Latin?) "levirate". Like: what are t'fillin? Those are "phylacteries". Gee, thanks. :-)

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My dictionary says, "Origin: early 18th century, from Latin levir, 'brother-in-law'"
The unhelpful translation issue comes up for me a lot in the context of sushi. E.g. it totally does not help to look up iwashi and find out that it's "gizzard shad".