Deuteronomy 25:5-7

If two brothers are living together, and one of them dies without having a son, his widow must not marry someone outside her husband’s family. Her husband’s brother must marry her, which is his duty to her as a brother-in-law. The first son she has counts as the son of the dead brother so that his name will not be forgotten in Israel. But if a man does not want to marry his brother’s widow, she should go to the elders at the town gate. She should say, “My brother-in-law will not carry on his brother’s name in Israel. He refuses to do his duty for me.”