Yeah, those people acted as if they'd never even thought about it before.

Personally, it didn't (and still doesn't) bother me because when I saw ESB in '82, we didn't yet know that they were siblings (and they probably weren't at that point), PLUS- it was pretty obvious that Leia kissed Luke only to make Han jealous.

It makes me all the more curious to see the previous scene (which was cut out of the film- a snippet remains in one of the trailers) when Luke & Leia are having a private discussion, lean in to kiss each other, and then are interrupted by 3P0.