Sasuke exists because Kishimoto needed a central character [img]i/expressions/face-icon-small-smile.gif[/img] Itachi exists as a motivational axis for that central character. Without Itachi, Sasuke would have to find other motivation, like having to prove his strength compared to that dropout Naruto. Since we already have had that kind of character with Neji, it's good that Itachi serves this purpose instead.

Itachi is the reason Sasuke strives for strength, but not the reason he exists. There's a difference.

In the end, they all exist because Kishimoto wanted it that way [img]i/expressions/face-icon-small-happy.gif[/img]