There is no need for any explanation on Spock's non metioned familial relationships.
The audience gets to know Spock has a cousin, sister, uncle, brother, pet cat or a human mother when the story requires it. The first episode of TOS did not establish Kirk had a brother until the story required him to have one. No one knew McCoy had a daughter until TAS, same with Sulu, and the audience did not need to know Chekov did not have a brother until Day of the dove.
I agree, except Kurtzman has made some sort of comment along the lines of "all will be explained" about why Micheal is never mentioned by Spock. Meaning he's planning for some sort of dramatic disjuncture by the end of the season.