However, that's not necessarily such an unusual thing. Spock was tight-lipped about his family, also not mentioning Sybok, his half brother, until the fifth movie, or who his parents were until the second season of TOS in "Journey to Babel". In both incidents, the dialogue makes it clear that his fellow crewmates, including his captain, were unaware of who his family were.
Dialogue from Star Trek V: The Final Frontier:
SPOCK: You do not understand me, Captain. Sybok, also, is a son of Sarek.
KIRK: He's your brother brother? You made that up.
SPOCK: I did not.
KIRK: You did too. Sybok couldn't possibly be your brother because I happen to know for a fact that you don't have a brother.
SPOCK: Technically, you are correct. I do not have a brother.
KIRK: You see?
SPOCK: I have a half-brother.
KIRK: I've got to sit down.
McCOY: Let me get this straight. You and Sybok have the same father but different mothers.
SPOCK: Exactly. That is correct. Sybok's mother was a Vulcan princess. After her death, Sybok and I were raised as brothers.
KIRK: Why didn't you tell us this before?
SPOCK: I was not prepared to discuss matters of a personal nature. For that I am sorry.
And from "Journey to Babel":
SAREK: Doctor. My aides and she who is my wife.
(He holds out his right hand with two fingers extended, and a human woman steps forward to touch them.)
AMANDA: Captain Kirk.
KIRK: Our pleasure, madam. As soon as you're settled I'll arrange a tour of the ship. Mister Spock will conduct you.
SAREK: I'd prefer another guide, Captain.
KIRK: As you wish, Ambassador. Mister Spock, we'll leave orbit in two hours. Would you care to beam down and visit your parents?
SPOCK: Captain, Ambassador Sarek and his wife are my parents.