Religion is an invisible prison. It makes us think we are okay with God, when we may actually be further from Him than the greatest of sinners. Sinners typically know they are sinning. Religious people never do.
And that’s the deal with the older son. He thinks he is better than his brother because he stuck around with dad. But he’s not better necessarily, for he is judging and condemning his brother, whom the father has accepted and forgiven! The older son is a lost son as well, and he too has turned away from his father.
Both the older son and the younger son need to see how God has graciously forgiven and accepted each of them, one for his many sins and the other for his religious hypocrisy, and both can thank the father for His love, and then show each other the same love, grace, mercy, and forgiveness in return.