Jacob speaks prophetically to his sons.
When he speaks to Reuben he exposes his son's sins. Sometimes it takes a harsh word from a loved one to awaken us to our sin so we may repent.
If we are not calling out sexual sin of every kind, we are not following God's law and not acting as people who care about others. If we look to Jesus we will see that He is not afraid to speak out against sin and He is also there to forgive.
Jacob warns Reuben that his sin has prevented him from excelling and will even affect the generations after him. Looking to the Book of Revelation we can see there is hope for Reuben and his descendents, and that God did not cast them away.