Genesis 35

"And so it was, as her soul was departing (for she died), that she called his name Ben-Oni; but his father called him Benjamin...And it happened, when Israel dwelt in that land, that Reuben went and lay with Bilhah his father’s concubine; and Israel heard about it." vv.18,22

Read chapter 35.
Rachel dies on their way to the promised land after giving birth to her son.  Rachel is the only main wives in Genesis that isn't buried with her spouse in the cave at Mamre with all the other patriarchs, we later see that Leah is, as she was Jacob's first wife.  Jacob calls this babe Benjamin meaning "son of my right hand."  This could possibly have upset or caused jealousy of Rueben and why we see him commit this offense that he did by sleeping with one of his father's wives (brother's mom).  Rueben being the first born would get the birthright and could've felt threatened by Benjamin so tried to take matters into his own hands by this act of dominance.  Ultimately this led to why he lost the birthright.  When have you seen this fight for power, especially in families?