Therin of Andor wrote:
But "Vulcan's Glory" has a line, from Amanda, IIRC, where Spock is called "the only son of Sarek". Years later, I asked DC about whether this was a deliberate stab at the then-not-yet-onscreen ST V storyline with Sybok and she smiled cheekily and said, "I'm glad you noticed."
Had the book been commissioned after ST V was out, that line would have been edited out.
Not necessarily. Sybok was disowned, after all, so officially he was no longer counted as a son of Sarek. Spock was the only one who had legal or familial recognition as a son after that point, and many people (including Kirk and McCoy) were unaware that Sarek had ever had another son.
But... it was possibly also because David Dvorkin's "Timetrap" was already in train. That one had Kirk apparently timeslipped into a future where the Klingons were the UFP's allies, IIRC, and it came out in June 1988. I recall, when reading it, that I'd hoped Dvorkin had been able to slip in a few TNG-type references, but there were none.
But because of the nature of the setup, there couldn't have been any.