“The duty of a brother-in-law”
When brothers dwell together and one of them dies without a son, the widow must not marry outside the family. Her husband's brother is to take her as his wife and fulfill the duty of a brother-in-law for her. (Deuteronomy 25:5 BSB)
What it sounds like today
An archaic rule forcing a man to marry his brother's widow.
The cultural key
This is levirate marriage: if a man died childless, his brother was to marry the widow, and their first son would count as the dead man's heir, preserving his name and keeping his land in the family. It served two deep ancient concerns, that a man's name not be blotted out and that a widow not be left destitute. The custom drives the story of Tamar and Judah, stands behind the whole plot of Ruth, and supplies the Sadducees with their trick question to Jesus about the woman married to seven brothers.
Recovered meaning
The law weds compassion for the widow to the horror of a name dying out, binding the family to provide both an heir and a protector. It is the quiet legal engine behind Ruth and a favorite puzzle thrown at Jesus.