I never understood why some people were against Spock having a half brother. Before TOS episode "Amok Time" we did not know about Spock's wife T'Pring and yet people don't complain about that, they accept it. Before this episode Spock never mentioned having a wife. So Spock having a half brother was not a problem with ST5 for me. I remember Majel Barret-Roddenberry complaining about us not knowing Spock had a half brother at a Star Trek Convention a year or two before the film was released.
Amok Time is the 1st episode of the 2nd season. The series was still new and things were still being established about Spock at this point. Episodes such as Amok Time, Journey to Babel, the mind meld, nerve pinch, his logic were clever things that established Spock's character.
In fact you can argue that of the main 3, Spock was the most established character (especially by the time of the 5th movie)
Now suddenly years later you suddenly add a long lost family member for Spock. It doesn't really add anything important to Spock. Okay it might explain why Sarek was so hard on him. But I think the fact that Spock was Sarek's only
child could explain that just as well.
Sybok dies at the end of the story but did having this 1/2 brother suddenly repeapper in his life and then die really do anything for Spock's character? I'd say no and that is my problem. Sybok for me adds nothing to the story except to add something that wasn't needed in the first place.
edit: As for Kirk's brother in Operation not a really huge fan of that either but we weren't discussing that episode. It was also still early in the series run when we don't yet know these characters as well.