Thy brother's wife - This was an illegal marriage, unless the brother died childless. In that case it was not only lawful for her to marry her brother-in-law, but he was obliged by the law, Deuteronomy 25:5, to take her to wife.
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Notes on the Whole Bible
Thy brother‘s wife - That is, if she had children. See Deuteronomy 25:5. The law here expressed was broken by Antipas in his connection with Herodias Matthew 14:3-4.