I’ve honestly disliked that line from the get-go.

Why say it within the context of this story? Luke is about to face down Vader. Don’t tell the audience Luke isn’t important anymore. Even if Lucas was thinking Luke should have a sister at the time, I don’t see why he would reveal that to the audience now.

It is clear to me Lucas originally wanted to have a new character be Luke’s sister, but when he decided to stop making Star Wars movies, he wanted to tie everything up into a neat bow. Happily ever after. So if Han and Luke are having a love triangle, just make Luke her brother! Tadaa!