In Jesus' day, "big brother" might have referred to the Pharisees, at least some of the more legalistic of them who were watching, looking down their noses, and casting judgment on "sinners". Their definition of "sinner" was "law breaker"; the assumption was that they, the "lawkeepers", were righteous. And some of them were angry with Jesus, who was accepting, even eating with, these "sinners". There is a "big brother" in the parable that should sound familiar to the Pharisees, perhaps to us?
Scripture: Luke 15:11-32