Leviticus 18:16 - Joseph Benson’s Commentary on the Old and New Testaments

Thy brother's wife Unless he died childless, for in that case God afterward commanded that a man should marry his brother's widow, Deuteronomy 25:5. For the prohibiting of marriages in the more remote degrees of consanguinity, where other moral considerations are less obvious, there is this good reason to be assigned, namely, that marriage being one of the firmest bonds of friendship, it is proper, for the greater good of society, that men should seek to enlarge the ties of friendship and social affection, by uniting, not with those to whom they were before related, but with persons of different families.