WHY did Sybok have to be Spock's brother?
Of all the problems this film has, this one bothers me the most. It's like something you would expect out of a daytime soap opera. "Hello Spock, I am your long, lost brother." *cue cheesy, dramatic music*
I would have accepted Sybok's character more if they just made him to be an emotional Vulcan who was an outcast in Vulcan society, and found a friend in Spock because of him being half-human and an outcast as well. Then they slowly grew apart as Spock gave in to the Vulcan status quo and Sybok pursued his religious quests.
While that sounds just as shoehorned-in, it's nothing like the big, fat retcon that Sybok was in the actual movie.