'Brother in law' in the Bible

Then Judah said to Onan, "Have sexual relations with your brother's wife and fulfill the duty of a brother-in-law to her so that you may raise up a descendant for your brother."

If brothers live together and one of them dies without having a son, the dead man's wife must not remarry someone outside the family. Instead, her late husband's brother must go to her, marry her, and perform the duty of a brother-in-law.

But if the man does not want to marry his brother's widow, then she must go to the elders at the town gate and say, "My husband's brother refuses to preserve his brother's name in Israel; he is unwilling to perform the duty of a brother-in-law to me!"