Deuteronomy 25:5-6
5 'When brethren dwell together, and one of them hath died, and hath no son, the wife of the dead is not without to a strange man; her husband's brother doth go in unto her, and hath taken her to him for a wife, and doth perform the duty of her husband's brother; 6 and it hath been, the first-born which she beareth doth rise for the name of his dead brother, and his name is not wiped away out of Israel.
Young's Literal Translation is in the public domain.
Matthew Henry's Commentary on Deuteronomy 25:5-6
Commentary on Deuteronomy 25:5-12
(Read Deuteronomy 25:5-12)
The custom here regulated seems to have been in the Jewish law in order to keep inheritances distinct; now it is unlawful.