The Torah gives us one sentence, and it is a scandal: Reuben went and lay with Bilhah his father's concubine (Genesis 35:22). The sages could not bear to leave it there. Reuben was Jacob's firstborn. He was supposed to inherit the priesthood and the kingship. Did he really commit so gross a sin?
Targum Pseudo-Jonathan on (Genesis 35:22) reopens the case. What Reuben actually did, says the Targumist, was move his father's bed. After Rachel died, Jacob had set up his sleeping mat in the tent of Bilhah — Rachel's handmaid — rather than in the tent of Leah, Reuben's mother. Reuben, grieving for the slight to his mother, rearranged the beds so that Jacob would sleep in Leah's tent instead.
The verse calls this as if he had lain with her. The Targum insists on that as if. It was a hot-headed, disrespectful act — but it was not incest.
Still, Israel heard it, and it broke him. He cried out: Alas, that one should have come forth from me so profane — as Ishmael came forth from Abraham, and Esau from Isaac! Every patriarch, it seemed, produced one rotten son. Was this Jacob's rotten son?
Then something astonishing happens. The Ruach ha-Kodesh — the Spirit of Holiness — speaks aloud, defending Reuben. Fear not, for all are righteous and none of them is profane. All twelve tribes, declares the Spirit, are pure. Reuben included. The line of Jacob would not be broken.
This is why the Torah lists the twelve sons immediately after: the count is affirmed, the tribe of Reuben stays in, and the whole nation of Israel — flawed, furious, grieving, human — remains holy.