I recall first seeing Sybok and being very, very annoyed. The actor didn't have the right build to be a Vulcan. My previous experience in Trek had led me to believe that stoicism (perhaps not logic, but stoicism) was something that the Vulcan/Romulan physiology lent itself to, and here was "a Vulcan" acting jovial. And I, like Kirk and McCoy in the movie, felt like I knew Spock well enough 
to know whether or not he had a brother.
Over time, though, I've come to conclude that:
1. Sybok was built like that because he had given up Vulcan dietary habits and embraced eating for pleasure as well as for need.
2. Maybe his lack of stoicism was also related in part to that - much like humans need serotonin to feel joy, there could be something akin to that in the Vulcan physiology that they just don't get a lot of in their chosen diet. Perhaps it is meat derived, and so they gave it up as part of their vegetarianism. Maybe there isn't even that much of it available on their planet anymore after the devastation caused by the various wars they engaged in pre-Reformation. Or it could be an intentional choice based on minimizing emotional responses in general.
3. It is, perhaps, only logical that Sarek would have bred with a Vulcan woman - presumably, he was betrothed to one at age 7 like most other Vulcans, and would go through Pon Farr as expected. If I recall correctly, the implication of the dialog in the film was that that was all *before* Sarek married Amanda, but honestly I can't see why that should be unless the female Vulcan died. Amanda would have to be a very tolerant person, but she *did* marry an alien, so she had to be open to things being just a bit different and difficult at times, too.
I think, also, that part of what Sybok was sharing *back* to people when he was sharing their pain was his own sense of freedom from the expected - letting them know that they don't *have* to be what they think they are. That's what I now take from the reaction of the first guy he did that to at the beginning of the film, anyway.
to know whether or not he had a brother.Over time, though, I've come to conclude that:
1. Sybok was built like that because he had given up Vulcan dietary habits and embraced eating for pleasure as well as for need.
2. Maybe his lack of stoicism was also related in part to that - much like humans need serotonin to feel joy, there could be something akin to that in the Vulcan physiology that they just don't get a lot of in their chosen diet. Perhaps it is meat derived, and so they gave it up as part of their vegetarianism. Maybe there isn't even that much of it available on their planet anymore after the devastation caused by the various wars they engaged in pre-Reformation. Or it could be an intentional choice based on minimizing emotional responses in general.
3. It is, perhaps, only logical that Sarek would have bred with a Vulcan woman - presumably, he was betrothed to one at age 7 like most other Vulcans, and would go through Pon Farr as expected. If I recall correctly, the implication of the dialog in the film was that that was all *before* Sarek married Amanda, but honestly I can't see why that should be unless the female Vulcan died. Amanda would have to be a very tolerant person, but she *did* marry an alien, so she had to be open to things being just a bit different and difficult at times, too.
I think, also, that part of what Sybok was sharing *back* to people when he was sharing their pain was his own sense of freedom from the expected - letting them know that they don't *have* to be what they think they are. That's what I now take from the reaction of the first guy he did that to at the beginning of the film, anyway.
