The following is the results of your search for levirate law.
Levirate Law: From Latin levir, "a husband's brother," the name of an ancient
custom ordained by Moses, by which, when an Israelite died without
issue, his surviving brother was required to marry the widow, so as
to continue his brother's family through the son that might be born
of that marriage
(Genesis 38:8; Deuteronomy 25:5-10) comp.
(Ruth 3:1; 4:10) Its object
was "to raise up seed to the departed brother."