The 'Lighter' Side Of The Force
Why did George Lucas let this even happen? He knew they were brother and sister? Or did he make it up later??
He just made it up.
The original treatment of "The Revenge of the Jedi" Lucas wrote "Vader says something to Luke to make him angry and attack" - as they got closer to shooting, the were trying to figure out what that would be. "The Other" was supposed to be Wedge (why he was a 'background' character, yet survived the first Death Star run, and Hoth, etc.) - but at some point they said "Luke and Leia are siblings!" :/