What sin did Reuben commit that cost him the birthright position?
Reuben, the eldest, committed sexual immorality by having a relationship with his father's wife or concubine. Because of that sin, God in His sovereignty gave the leadership position Reuben should have enjoyed to Judah instead. He is described as an immoral person, and his sin disqualified him from the place his firstborn status would have carried.
Les teaches that Reuben was the oldest, so he should have occupied the lead position, but Reuben sinned, and God in His sovereignty brought Judah to the place Reuben should have enjoyed. Les notes that Reuben was an immoral person—he had a relationship with his father's wife or concubine or something. Therefore Judah takes the leadership, and the other reason is that Christ comes from Judah; the scepter shall never leave Judah. In the camp arrangement, Judah led on the east, while Reuben was on the south with Simeon and Gad.