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 (Gen. 38:8; Deut. 25:5-10;
compare Ruth 3; 4:10). Its object was "to raise up seed to the
departed brother."