You sure?
Where No Man... said:
KIRK: Have I ever mentioned you play a very irritating game of chess, Mister Spock?
SPOCK: Irritating? Ah, yes. One of your Earth emotions.
KIRK: Certain you don't know what irritation is?
SPOCK: The fact one of my ancestors married a human female
KIRK: Terrible having bad blood like that.
He openly mentions having a human ancestor, yet no mention of a human sister. He also openly talks about his human mother during "The Corbomite Maneuver" during season one.
At the very least, the intent is clear. Couple that with DC Fontana's feelings (she was one of the people who helped define who Spock was) and I think you come about as close to "canon" as can possibly be without being "canon". The connection to Spock was nothing but a way to hook audiences.