After finding out Vader was Luke's father when I was five, and discovering Leia was Luke's sister in ROTJ, I just figured everyone in Star Wars is related.
This led me to think that Han was Luke's brother.
Come to think of it, when I first heard the Vader/Luke revelation, I thought it too far-fetched.
For a long time, I was convinced that Vader was lying and for some reason Obi-Wan was Luke's father.
And then I saw the prequels.