Genesis 35:22

22While Israel was living in that land, Reuben had sexual relations with
Heb “and Reuben went and lay with.” The expression “lay with” is a euphemism for having sexual intercourse.
Reuben’s act of having sexual relations with Bilhah probably had other purposes than merely satisfying his sexual desire. By having sex with Bilhah, Reuben (Leah’s oldest son) would have prevented Bilhah from succeeding Rachel as the favorite wife, and by sleeping with his father’s concubine he would also be attempting to take over leadership of the clan – something Absalom foolishly attempted later on in Israel’s history (2 Sam 16:21–22).
Bilhah, his father’s concubine, and Israel heard about it.

Jacob had twelve sons:

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