There are two people named Shelah in the Old Testament.
Shelah the Younger was the son of Judah and Shua the Canaanite. Tamar had been the wife of Judah's elder sons Er and Onan (via Levirate marriage; cf. Deuteronomy 25:5-10). After Onon died, Judah refused to let Tamar marry Shelah. Tamar then bore twin sons (Perez and Zerah) by Judah, after disguising herself as a prostitute. (Genesis 38, 46:12). Although biologically Judah's sons, Perez and Zerah were legally Er's sons; however, Leviticus 18:15 forbids sexual relationships between a man and his daughter-in-law.
This Shelah became the ancestor of the clan of the Shelanites, (Numbers 26:20; 1 Chronicles 2:3) and had at least two sons: Er and Laadah (1 Chronicles 4:21).