The midrash (Bereshit Rabba 84:19) indicates that Reuben did teshuva (repentance) for sleeping with his father's concubine Bilhah (Bereshit 35:22). However in both Yaakov's blessing (ibid 49:3-4) and in 1 Divrei HaYamim (5:1) Reuben's sin is discussed as if his act teshuva had not gained him forgiveness. I would like to know if my assumption that Reuben's sin was not forgiven is correct, and if so why not. If his repentance was in fact accepted why was he still described as being a sinner.
Rav Hirsch explains that Yaakov first turns to determining which of his sons will become the leader of the family. As such he discusses the character traits that will lead to the malchus (kingship). This means that he is saying that even though Reuven repented of the actual action, the character flaw that led to it still disqualified him from becoming the king.
This flaw was that he was unstable as water. Another translation of Vay'chi 49:4 is
[You have] the restlessness of water; [therefore,] you shall not have superiority,
Rav Hirsch explains:
But as nevertheless he says here: "you are the most precious gem in my treasury, are כחי וראשית אוני, but for the leadership of the family you are not qualified, for that one must be עז, may not be moved by soft flattering winds nor by stormy hurricanes, must have strong inner firmness not be unstable like water", from all this it was evident that the incident referred to could not have had that importance which the harsh blunt words וישכב וגו would seem to imply.
Reuven had a tendency to jump up and act hastily, as we see when he offered to allow his two sons to be killed if he failed to bring back Binyamin. He rushed off, planning to rescue Yosef, without thinking that others (like the Midyanim) might come by and pull Yosef out and sell him. In fact he moved his father's bed from Bilha's tent to Leah's because he was sensitive to the honor of his mother. This is what is regarded as his sin even though it is not the literal meaning of Vayishlach 35:22
And it came to pass when Israel sojourned in that land, that Reuben went and lay with Bilhah, his father's concubine, and Israel heard [of it], and so, the sons of Jacob were twelve.
As Rashi says:
and lay: Since he (Reuben) disarranged his (Jacob’s) bed, Scripture considers it as if he had lain with her. Now why did he disarrange and profane his bed? [It was] because when Rachel died, Jacob took his bed, which had been regularly placed in Rachel’s tent and not in the other tents, and moved it in to Bilhah’s tent. Reuben came and protested his mother’s humiliation. He said,“If my mother’s sister was a rival to my mother, should my mother’s sister’s handmaid [now also] be a rival to my mother?” For this reason, he disarranged it. — [from Shab. 55b]
and so, the sons of Jacob were twelve: [Scripture] commences with the previous topic (i.e. the birth of Benjamin). When Benjamin was born, the marriage bed (i.e. the destined number of sons) was completed, and from then on, it was proper that they be counted, and [so] it (Scripture) counted them. Our Sages, however, interpreted that these words are intended to teach us that all of them (Jacob’s sons) were equal, and all of them were righteous, for Reuben had not sinned. — [from Shab. 55b]
Rav Hirsch emphasizes that this was a character flaw that prevented leadership rather than an actual sin.
To a son who was the כח וראשית אונו of his father and who should be יתר שאת ויתר עז any offence which would otherwise be considered quite lightly, is taken most seriously. Had he really committed a real sin, surely a Jacob would not have designated a Reuben by such a mild and gentle epithet as פחז כמים.