screams in the void said:
If you watch the documentary From Star Wars To Jedi , Lucas talks about how he needed a reason for Luke to snap on Vader when hiding under the stairs and until that moment , he did not realize it was staring him in the face in form of Leia being his sister . So that was when that plot point came along .
If the plot point could come to him at that moment (out-of-the-blue), how is it less probable for the notion to have occurred to him years earlier?