I can't see how the existence of Sybok affects how Spock is depicted...
Do you have brothers and sisters? Would
your life have been quite different if you were an only child?
DC Fontana and Gene Roddenberry nurtured the growth of a character who was being written by many hands. They specifically asked writers on TOS
not to add siblings to the mix for well-considered reasons.
True, but there's no reason for that to be set in stone. And an artistic decision made back in 1966, while producing a weekly TV show, is certainly worth revisiting when making a feature film two decades later. You need to allow for some flexibility over the course of decades. And changing creative hands.
And while, abstractly, siblings affect character, it's hard to point to how exactly the existence of Sybok changes our understanding of Spock. Indeed, in some ways, you could argue that this revelation actually fleshes out Spock even more.
Take "The Way to Eden" for example. Spock's sympathy for the space hippies makes more sense if you retcon Sybok into the equation. No doubt the rebellious pilgrims, seeking some mythical paradise, reminded him of his long-lost half-brother . . . .
Or look at "Journey to Babel." Sarek's rather extreme reaction to Spock joining Starfleet (seriously, he didn't speak to his son for eighteen years?) makes more sense once you realize that Spock is the
second of Sarek's sons to seemingly turn his back on Vulcan. No wonder Sarek took it so badly!
See what I mean? Rethinking your premises doesn't necessarily damage what was established before; done right, it can just add new facets to the characters.
(Not that anybody is really defending ST V!)