If you think this needs to happen in order for TOS Spock to never have an adopted sister simply because he never talked about having an adopted sister, then why don't you feel the need for Sybok to do something similar, considering that Spock never spoke of Sybok either until ST:TFF.
If you want an "in-universe" reason why Spock never spoke of Burnham or Sybok on TOS, you don't need to look any further than the episode "Journey to Babel". In that episode, we see that Spock is so secretive about his family that he was more than happy not telling anyone -- not even his best friends -- anything about his parents, even when his parents were standing right in front of Kirk. I think Spock would have been fine with not telling Kirk that Sarek was his father and Amanda was his mother if Kirk did not ask Spock if he wanted to visit his parents while they were at Vulcan anyway.
So considering that scene, it was not be surprising that Spock had a half-brother Sybok who he never mentioned before, and it would not be surprising if he had a sister he never mentioned before. The reason for us not knowing about them is already consistent with "Journey to Babel", so there is no need for another explanation.
