I had an insight into this story with regards to two of my dogs. The new one demanded attention so that the old one became neglected. It's attention addiction and the whole thing is about balance - that is inclusion and adjustment. At its root is jealousy* on one side (the loyal child) and overcompensation on the other (the father), plus the urge to keep the family (or race) together, through forgiveness and favouritism - at least that is how I see it.
* Hence the brother questioning the father about his actions, instead of accepting them.