Now the sons of Reuben the firstborn of Israel, (for he was the firstborn; but, forasmuch as he defiled his father’s bed, his birthright was given unto the sons of Joseph the son of Israel: and the genealogy is not to be reckoned after the birthright.
Now, Reuben's sons, who was the first child of Israel, had a birthright. But because he did something wrong in his father's bed, the birthright was given to Joseph's sons instead. So, their family line is not counted based on the birthright.