This type of union, known as levirate marriage (the English word comes from the Latin term meaning “brother-in-law”), is designed to prevent the dead husband’s lineage from dying out: His brother impregnates his wife in order to provide him with a posthumous heir. The practice is commanded in Deuteronomy 25:5: “If brothers dwell together and one of them dies and he has no child, the wife of the dead man shall not be married outside of the family to one not of his kin; her brother-in-law will have intercourse with her and take her to him to be his wife and consummate the levirate marriage.” There is, however, a significant problem with this commandment, because it seems to directly contradict an earlier prohibition from the book of Leviticus: “You shall not uncover the nakedness of your brother’s wife; it is your brother’s nakedness.” When two mitzvot conflict, the Talmud asks, how do we decide which takes precedence?As they say on Facebook, it's complicated.
Earlier Daf Yomi columns are noted here and links.