Cleopatra said:

I think Leia's line is less of a "I've known all this time" and more of a "I've always felt closer to you" line. And in all honesty, it's not that odd. I remember several news stories from the past few years about brothers and sisters who were separated at birth and ended up getting married! It wasn't until blood testing was done that they learned the truth. So maybe familial bonds really do attract people, even if they don't understand why the attraction exists.

I posted a link to such a case in defense of the whole scenario which for an eleventh hour quick fix actually works rather well with what we already know from ESB but consider a similar famous cinematic situation.

When Marty McFly gets face sucked by his mother in Back To The Future she instantly pulls back (even though she was very keen up until that point).

She instinctively knows something is wrong with that situation and she isn't a Jedi.

If Leia "somehow always knew" Luke was her brother her lipsmackin' routine in ESB becomes rather laughable (and not in a good way).