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1.
Sometimes a pain in the ass, sometimes someone you never want to talk to again, but other times can make all the bad things that happen go away when its all said and done.
Big sis: Why are you so sad??
Lil sis: Big sister the kids at school are mean
Big sis: aww well I love you!
by yhctirt May 31, 2009
 
2.
Government surveillance efforts that utilize surrepitious methods and deny any involvement because they lack the testicular fortitude to defend their own actions.
"Red light cameras are classic Big Brother, but NSA scanning of Google Earth footage is totally Big Sister."
by RandalPnk October 01, 2007
 
3.
A crazy female teacher who is "always watching you".
Derrived from George Orwell's novel Nineteen Eighty-Four, where a character named "Big Brother" and his "Thought Police" are keeping track of everything and everyone.

Big sisters tend to be severe in punishment as well.
Big Sister:

Mme. Zylberstein.

Any crazy old female teacher you had who appears to have eyes in the back of her head.
by ArcticWarfare April 02, 2006