'Eldest' in the Bible

The sons of Judah: Er, Onan, and Shelah; the three were born to him by Shua’s daughter the Canaanitess. Er, Judah’s eldest, was evil in the Lord’s sight, and He put him to death.

Penuel was the father of Gedor, and Ezer the father of Hushah. These were the sons of Hur, the eldest of Ephrathah (Ephrath), the father of Bethlehem.

Now [we come to] the sons of Reuben the firstborn of Israel—for Reuben was the eldest, but because he defiled his father’s bed [with Bilhah his father’s concubine], his birthright was given to [Manasseh and Ephraim] the sons of Joseph [the favorite] son of Israel, so that he is not enrolled in the genealogy according to the birthright.

These individuals also cast lots corresponding to their relatives, Aaron's descendants, in the presence of King David, Zadok, and Ahimelech, and in the presence of the heads of the ancestral households of the priests and of the descendants of Levi, and the eldest was treated as impartially as was the younger brother.