It is important to note that the parable ends with a conversation between the father and the older brother. It is significant that Jesus is leaving his audience not with the celebration of the younger son’s return (as important as that is) but with the resentment and jealousy of the older brother. In many ways the parable could be called the parable of the prodigal sons because although by worldly standards the younger brother seems to have fallen farthest it is the older brother who seems most distant from his father in the end (and in fact may never have been as close as he thought).
To best understand why Jesus puts the emphasis on the older brother in the end when you would expect it to be on the younger you need to go to the beginning of the chapter and see who Jesus is actually speaking to:
Really a brilliant lesson with new facets every time it is read.