Who is he is someone who denies his human side at almost every turn for many years. I do not see the contradiction in terms of the psychology of Spock.
My issue here is that there are usually differences between the products of families with only-children, larger nuclear families, and non-standard family groups. (This is even referenced regarding Harry Kim in the Trek canon.)
Ideally, such differences wouldn't exist much in flawlessly logical Vulcan families, but of course that's not even a thing.
To be sure, we already knew Vulcan children could be asses to the half-Vulcan child Spock, as JJ-Trek and TAS both put on-screen based on Amanda's commentary in TOS. Star Trek V gave us a live-in older half-brother for a time, but we know very little about that situation. Its duration could've been minimal.
We can guess at some results… perhaps Sybok reinforced Spock's Vulcan choices by serving as a negative example, and Amanda may have played a final role in encouraging them both (which backfired). Perhaps there was a 'living in the shadow' effect given what a scholar Sybok was reputed to be.
However, generally speaking, there's not necessarily a great change required from previous "only child" outcast status.
However, adding a literal human being of similar age changes a lot of things. For one, Spock is no longer the lonely outcast, as there's a literal alien *right there* to serve as any number of things… protector of, bully to, blame target, competitor, et cetera. It's one thing to see his Vulcan father and human mother's dynamic and replicate that in his own soul, but to then have his own human to observe and play off of . . . one apparently as successful at being Vulcan as he was . . . is a game-changer.
To be sure, wargaming someone's resultant philosophy and life choices based on adding or subtracting major variables is by no means a simple mathematical equation, and there are as many hypothetical conjectures and as much room in the statistics as needed to disregard the point, but, for many, those would be gaps that the second season would need to do a damn good job filling in for the sake of believability.
And yes, we can attack other Trek at this point by referencing Sisko's largely-missing sister or other things, but the issue is that the STD writers, assuming they fail to adequately address the change they've wrought, are monkeying about with an iconic character. That's not something most of us would do lightly.