en_tn_condensed/1ch/05/01.md

but his birthright was given to the sons of Joseph son of Israel

"but Israel gave Reuben's birthright to the sons of Joseph, another of Israel's sons"

Reuben had defiled his father's couch

This is a polite way to speak about Reuben sleeping with his father's secondary wife. The couch is the place where a man and his wife would have slept together.

he is not recorded in the genealogy as having the birthright

"the family history does not list Reuben as the oldest son"