It is plausible....hold on, that was pretty good. Perrin!
Yes, say Perrin had a son before marrying Sarek (she's widowed, whatever).
Now when she wed the ambassador, perhaps Perrin's child became a "son of Sarek", adoption, Vulcan customs, or something like that.
When Picard mentions meeting Sarek at his "son's wedding", he might be referring to Sarek's step-son (son of Perrin).
The step-son concept might not translate in Vulcan tradition or custom.
Spock didn't refer to Sybok as a step-brother, just a brother.
So Picard "Sarek's son" comment might be referring to his step-son by Perrin?
Am I missing something? Plausible?
But Sybok was not Spock's step-brother. That would mean that they had no blood relation. He was his half-brother - they had the same father - and most people, humans included
