If.

Genesis 38:8
Then Judah said to Onan, "Sleep with your brother's wife. Perform your duty as her brother-in-law and raise up offspring for your brother."

Deuteronomy 25:5-10
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. . . .

Ruth 4:5
Then Boaz said, "On the day you buy the land from Naomi and also from Ruth the Moabitess, you must also acquire the widow of the deceased in order to raise up the name of the deceased on his inheritance."

that.

Ruth 1:11-13
But Naomi replied, "Return home, my daughters. Why would you go with me? Are there still sons in my womb to become your husbands? . . .