This is levirate marriage (from Latin levir, “husband's brother”; in Hebrew yibbum): when a man dies childless, his brother marries the widow, and the first son “shall succeed to the name of his brother who is dead.” The same passage provides the release ceremony (chalitzah, the loosing of the sandal) for a brother who refuses. The book of Ruth turns on this family duty, and the tradition built a whole tractate of the Mishnah, Yevamot, on its cases.

This law is the machinery of the Sadducees' riddle in Matthew 22:23-33: seven brothers, one widow, “in the resurrection, whose wife shall she be?” The riddle only works because the duty was real law. Jesus answers the question behind the question, proving the resurrection from the Torah itself (Exodus 3:6), the one canon His questioners fully granted.