While a number of longtime fans were miffed about Star Trek: Discovery introducing an adopted sister that Spock has never mentioned, this isn't the first time he's had a sibling he never told people about.
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.
We learn a little about Spock's childhood in Star Trek Discovery. Michael notes that her foster mother, Amanda Grayson, read to "me and her son" (Spock), meaning that Sybok wasn't being read to.
When I was a kid, after my parents were killed my foster mother on Vulcan used to read it to me and her son. She and I were the only humans in the house.
–Star Trek Discovery, "Context is For Kings"
Furthermore "Lethe" involves Michael's childhood, in which Sarek and the Director of the Vulcan Expeditionary Fleet discuss the future of his children, but not once is there any reference to Sybok (or any additional children).
Sarek: I have created in her a being of exquisite logic to rival the best of our species.
Director: An accomplishment you have achieved not once but twice. Your ward, Michael, and your son, Spock.
–Star Trek Discovery, "Lethe"
Where is Sybok during these events in Michael and Spock's childhood? His absence seems to be at odds with Spock's statement that they were "raised as brothers".