Birnbaum isn't Spock's half sister, she's just his adopted sister.
Cue the hecklers in the crowd shouting "why wasn't she namedropped in 'Amok Time' or 'Journey to Babel' or 'The Search for Spock' or 'The Voyage Home' or 'Unification' or..."
She's also the first human to attend the Vulcan Science Academy. Putting aside the novels saying otherwise (and there are some good examples of humans choosing to live among Vulcans among them), I find the notion ridiculous. Either the Vulcan Science Academy teaches things concerning the unique mental powers and spiritualism of Vulcans, in which case they would never let humans in since they couldn't participate fully, or else it's just a college on another planet, in which case humans would be asking to attend way before now.
Why Sarek adopted her.
So the Klingons killed her parents when they were Sarek's guests on Vulcan. The words "bad fanfic plot" keep appearing in my head, I wonder why...
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mudshark: Nate's just being...Nate.
Zeke: It comes
nateurally to him.
mudshark: I don't expect Nate to make sense, really -- it's just a bad idea.
Sa'ar Chasm on the 5M.net forum: Sit back, relax, and revel in the insanity.
Adam Savage: I reject your reality and substitute my own!
Hanlon's Razor: Never attribute to malice that which can be adequately explained by stupidity.
Crow T. Robot: Oh, stop pretending there's a plot. Don't cheapen yourself further.