The oldest, like all oldest brothers, is somewhat envious of the spoiling the younger two receive, and often thinks he's too mature to hang out with his younger brothers' friends---even when he does, he doesn't understand half the new toys and TV shows they're talking about. But when they need help, he's often there; and he's always willing to share his experience in life to help them, if they'd just wait until he was done with what he was doing.
The middle brother is envious of both his older brother, already with the privileges of age like "use of the car" and "a later curfew"; and the younger brother, who's spoiled to the extent of being given freely toys the middle brother had to deliver newspapers for. Occasionally he gets bitter about how both the others seem to have it better, and goes into a funk about how Father dislikes him, but usually he's happy to work together with the others, although the older's constant talk of dates and cars annoys him.
The youngest brother is still little, but makes a point of being "closer to the middle than the middle is to the oldest", and big for his age anyway, so often says that he's about as old as the middle brother. He's annoyed when he's thought of as as young as he is. He hasn't matured enough yet for anyone to be sure how he'll interact with the other two when he's grown.