What about TWOK? It gave Kirk a never-before-mentioned adult son, along with the lost love who was his mother (although lost loves/old flames out of nowhere were a standard TV trope that TOS used multiple times). It also gave Spock a longtime protegee, Saavik, whom we'd never heard of before. And in the Director's Cut, it gave Scotty a nephew. TWOK probably sets some kind of record for the number of relationships-out-of-nowhere it introduced.
And McCoy's daughter wasn't mentioned onscreen until the animated series. "The Way to Eden" would've been the story that introduced her, but it was rewritten heavily and she became Chekov's old girlfriend instead.
Once again my main issue with Sybok is he is a long lost family member that becomes so integral to the 5th movie but in the end really doesn't add much of importance to Spock's character. In fact he feels forgotten by the end of the movie. Sybok doesn't add depth to Spock's character, it just feels like a twist.
I am not saying suddenly introducing family members is always a bad thing but Spock already had an established family in the show: characters like McCoy and Scotty did not.
I am also not a terrible big fan of the way Kirk's son is suddenly introduced in WOK and then killed off in the next movie. Surely Kirk would care about his son dying but the movie didn't make me care. But at least this can explain why Kirk has a hard time trusting Klingons in the 6th movie.
Sybok becoming Spock's brother happened mainly because Shatner/Loughery had written themselves into a corner with the scene in the shuttlebay where Spock has Sybok at gunpoint. Up to that point they'd had Sybok merely as a childhood friend/contemporary of Spock. But there was no good reason why Spock couldn't/shouldn't shoot Sybok at that moment and protect his crewmates, unless...
as McCoy later put it, "He could no more kill his brother than he could kill (Kirk)."