All throughout Star Trek: The Original Series and up through Star Trek IV: The Voyage Home, there was no mention of any siblings for Spock. Even when he had died and returned to Vulcan to restore his katra , there were no brothers or sisters mentioned. That was because at the beginning of the second season of The Original Series, writer D.C. Fontana told the fanzine Spockanalia [via Slashfilm] that not only had Sarek only been married to Amanda, but there were no other children. Spock was an only child.
Star Trek V: The Final Frontier was the first to break Fontana's rule when Sybok was introduced as Spock's half-brother. Supposedly, he was Sarek's son with a Vulcan princess, and there was no mention if the pair had actually been married. Fans weren't pleased with the introduction of this never-before-mentioned half-sibling, but, of course, the internet wasn't around at the time so they couldn't let their displeasure be heard as much as today.
Enter Michael Burnham in Star Trek: Discovery, the adopted sister of Spock. That set the fans abuzz, and very few of them were happy about this latest revelation. Many claimed there was an issue because there was no previous mention of her. But there wasn't a previous mention of Sybok, either. Spock didn't even immediately tell Captain Kirk and Dr. McCoy who his parents were in "Journey to Babel." On top of that, Michael and the entire crew of the Discovery could not be mentioned again once they traveled to the 32nd century. And since Discovery was set ten year before The Original Series, it makes sense that Spock would not mention her.
To this day, though, there is little issue with Sybok and so much more with Michael. Was it easier for fans to believe that Spock had a sibling back in 1989 than it was in 2017? Or did fans think, after Sybok, Spock would have mentioned the rest of his family to Kirk?
Actually, it's probably more that Michael didn't haven't to be a part of Spock's family to make Discovery work. Many fans felt like the producers were simply trying to shoehorn her into The Original Series. But, as has become clear now, the connection to Michael and Spock was a great introduction to Captain Pike and set up the spin-off, Star Trek: Strange New Worlds. That has definitely appeased some of the fans. Still, many will never accept Michael Burnham as Spock's adopted sister, but when you think about it, we don't actually know what other members of Starfleet have family members they haven't mentioned. So even Captain Pike could have a couple of sisters we may meet down the road.