I haven't noticed if anyone else has noted this but Luke does not talk about the stories in Luke 15 as being three parables. In 15:3 it states "He told them this parable..", but in the rest of the chapter, each of the other stories is not called a new parable. I believe they are all together as three different aspects of the same parable, a harmony or chorus written as a trilogy under one suggested heading That Which Was Lost, and Was Found.
Since this parable was not directed at His disciples, but rather at the tax-collectors, sinners, Pharisees and scribes, I don't think we should interpret it as being for Christians. Although there are aspects of reconciliation there, the parable seems to be about the "Lost and Found" and the joy of the One who is the finder.
Both sons were "lost" but only one realized it, and admitted it. He was restored. There is no suggestion that the older brother was willing to admit his sins. He was dishonoring to his own father. He was self-righteous in his suggestion that he "never neglected a command of yours". This echoes the words of that unfaithful young man from Matthew 19:20.
So since this parable was expressed to the Pharisees, I suggest that He was trying to show them that their own unrighteous arrogance made them blind to their need. Unlike the sinners and tax-collectors who "drew near to Him" (15:1), these men were scrutinizing Him, "watching Him closely" (14:1) and "plotting against Him, to catch Him in something He might say" (11:54).
It's ironic that they murmured quitely under their breath about how Christ received sinners "and eats with them". Christ had a meal with the Pharisees, just a few hours before! (14:1)