What terrible sin did Reuben commit?
Reuben, Jacob's eldest, committed adultery with one of Jacob's wives. This was described as the most unchaste, promiscuous, and awful deed among the twelve sons. Because of it, Jacob later declared Reuben would not receive the birthright or blessing and would not be esteemed as the eldest.
Les explains that Reuben, the eldest, was the most unchaste and promiscuous of Jacob's twelve sons and did the most awful deed: he committed adultery with one of Jacob's wives. Evidently nothing was said of it until old Jacob's deathbed. As Jacob began pronouncing blessings on his twelve sons, he started with Reuben and brought the sin to mind, declaring that Reuben would not have the birthright or the blessing and would not be esteemed as the eldest because of that adultery. Seemingly no one else ever knew it.