Even though it likely wasn't the intention, I always felt that Luke calling for Leia with The Force at the end of ESB was a hint of that familial link. Considering there had been this slight underlying "maybe" attraction between Luke & Leia, I suppose this was another suggestive hint. Still, it works better as a possible retcon idea. Vader reveals he is Luke's father, and then, the next (living) person he calls for help is Leia, who happens to be his sister. It strengthens the theme of family bonds in the saga like Anakin's nightmares about Shmi & Padmé. So, the whole line about, "I've always known," does fit for me. If you push it further to that Leia never actually met her mother, but had visions of her through The Force, it fits even better. It means that she always knew there was a special power she had, or an extraordinary bond with her biological family.

-NJM