This is another topic that I formulated an opinion about whilst reading another user's blog but I have been thinking about it since my first viewing of ROTS.

The issue that plagued my thoughts immediately after seeing ROTS dealt with Leia's memories of her mother (Padme) as described by her in ROTJ. Although the time period bewteen the birth of the twins and Padme's death is not specifically implied, I do not deal with this detail because it cannot be assumed that it was a long interval any more than it can be assumed that it was a short one. I have formulated an explanation that takes what is known and couples it with what is plausible, resolving the situation in an acceptable way (or at least an acceptable way for me).

I believe that the emotional memories Leia has were imprinted on her when she sensed her mother following her birth, her force sensitivity and Padme's strong feelings at that time only heightening this connection. I also think it is safe to assume that Bail did mention Padme to Leia as she grew older.

In Leia's position it would have been relatively safe to disclose such information (in comparison to Luke as he is the male and only one child was thought, by Palpatine and Vader, to have been born). Bail Organa's personal relationship with Padme would have inspired such talk as well, whereas Beru and Owen didn't have the luxury of knowing Padme intimately. This combination, to me, seems a perfectly logical explanation for Leia's ability to speak on the matter upon Luke's inquiry.