What about pictures? As in photos, holosgrams, whatever? Leia would constantly be aware that she was an a dopted child, at least from her early teens and on ('cos the Organa's are black, see? Genetics still holds even in the PT). It would only make sense for her parents to prepare her a bit for it, maybe by showing her images and telling her about her real mother - they would obviously keep schtum about her father, due to him being the person who hunts down and kills Jedi all over the galaxy. Anyway, I dunno about all of you, but when it comes to my earliest recollections, I have a hard time knowing what I actually remember and what has been told to me afterwards, later on in my childhood.
Cannot Leia's recollection simply be explained with the fact that her parents had access to imagery of Padmé and knew her well to want Leia to know her? Owen and Beru never met the real parents, are most likely quite intimidated by the situation. They would not want to complicate things by telling Luke stories of a royal past and thus make up the stories about a less glorious father, possibly with some input from Obi-Wan.
Of course, this bring us to another plothole... How the heck didn't someone in the Imperial camp notice that Leia (who obviously isn't the biological daughter of the Organas) looks a wee bit like her mum and dad while she runs around the senate, whipping up support for the Alliance? She's not the kind of politician you don't notice, at least not as far as we know.
