In 1989, I wonder how many people reacted to Spock's newfound family tree addition, and if they were for or against it. It was out of the blue, certainly, and TNG continued the trope that Sarek kept marrying on after Amanda. But it's not often a trope that manages to remain so usable. All of a sudden, everyone has more family members than Dorothy, Rose, Blache, and Sophia combined. Sometimes the same family member being played by a different actor. At least with sitcoms, continuity isn't as much concerning, and even in some cases, is used as fourth wall-breaking joke fodder*.
Keeping in mind that the following scene is set in the brig:
McCOY: Let me get this straight. You and Sybok have the same father but different mothers.
SPOCK: Exactly. That is correct. Sybok's mother was a Vulcan princess. After her death, Sybok and I were raised as brothers.
KIRK: Why didn't you tell us this before?
SPOCK: I was not prepared to discuss matters of a personal nature. For that I am sorry.
KIRK: That makes everything all right? I'm sorry...
McCOY: Stop it, Jim. Spock could no more kill his own brother than he could kill you. If you want to punish him for what he's done, why don't you throw him in the brig?
A shame they didn't say when Amanda died, or Amanda raised both Sybok and Spock, or if Sarek prior to that was pulling a Three's Company stint and tending to both marriages simultaneously. Whatever the logical reason is, of course.
But it's still better continuity than the "'Amok Time' vs 'The Cloud Minders'" issue was where the deeply personal thing Spock didn't want to talk about is now rambled upon to every stranger with a navel that shouldn't have been shown on screen in 1969, yet Spock remains as stoic and unemotional as opposed to being excited to tell the universe he can get jiggy too and will prove it, it's not a prerequisite for the captaincy like what Kirk and Riker do all the time or anything. Not sure what the reason was for television at the time, it's not the sort of spatial black hole collapsed star where the Enterprise goes through it and ends up in another dimension or anything...
* Case in point: Aunt Viv 1 > a pot of spaghetti noodles > a bowl of dust mites > Aunt Viv 2
