Genesis 38:11

Remain a widow. The sudden death of his two older sons, so soon after their marriage to Tamar, made Judah hesitate to give her the third as a husband. In harmony with a superstition found in the Apocryphal book of Tobit (ch. 3:7-10), he may have thought that either she herself, or marriage to her, had in some way occasioned the deaths of Er and Onan. Therefore he sent her away to her father’s house, with the promise of his youngest son as soon as he had grown up. That Judah never intended to fulfill his promise is clear from his excuse that Shelah might “die also, as his brethren did.”

When Shelah had reached a marriageable age but was not given to her, Tamar determined to secure a child by Judah himself. This was completely in harmony with prevailing Hittite and Assyrian custom. The laws of the Hittites and Assyrians contained the provision that the duty of levirate marriage was to be performed by the father of the deceased if no brother was available.