If young tos Spock meets kelvin Spock, and interacts with him too much, it may influence tos Spock life's a bit and change his story rewriting Nimoy's version.
For instance, kelvin Spock has resolved most of his mixed human/vulcan conflict while it took tos Spock many years to do that..and disco-Spock presumably is just at the beginning...
Tos Spock didn't see his planet getting destroyed, didn't have to feel survivor guilt, didn't see his mom die, didn't have a girlfriend that challenged him to embrace his feelings and dual heritage all the more. Even their goals and desires, their perspective on things, might be very different.
They'd have to retcon that into a 'whatever happened, happened' situation like in Lost. By the time disco Spock becomes Spock prime he'd essentially meet kelvin Spock again knowing (or without knowing for sure) that he's his alternate self he had met when he was younger. Fascinating.
BTW, the comics already had a little crossover where the kelvin crew found themselves in the tos reality and viceversa. And the tos characters saw some of the differences.. You could say that crossover retconned tos because it inevitably adds the notion that the tos characters knew about the existence of this other reality and how different they are.
An interesting thing for me is that I don't think these characters really perceive each other as merely a version of themselves. The way Kelvin Spock grieved the loss of Spock prime in beyond was more like he lost a father figure, or a grandpa. .an old guy he cared about. I don't think he perceived that Spock as himself, and probably the other characters too saw him as his own separate person.
The real question for me is: does kelvin Spock have a sister, thus Michael, too? Or maybe her parents never died here and thus Sarek never adopted her.
Or maybe she exists here too but he didn't mention her either.
However, Spock wouldn't be the only one who has siblings not mentioned in the movies so rather than explaining why he didn't mention them, the answer might be simpler like: the characters (eg. Uhura, Kirk..) know about her, and know about the families of the crew, it's just that those aspects aren't talked about in the movies because there isn't a pretext to do that. After all, we know, for example, that Sulu has a partner and daughter because we saw them, not really because he talked about them with his friends or they mention his family (and it's implicit that his friends know about them).
