It wasn't, actually. Not entirely anyway. But I won't ramble on about it here, since it would probably bore all other (save for the two other abovementioned) people to death... The answer to the question is, of course, that Breha didn't yet exist when Barbara Hambly wrote Children of the Jedi and came up with Leia's three dowager aunts... Seriously though, Lucas said himself in The Annotated Screenplays that he wanted one of the Skywalker twins to remember their real mother, and it's been stated elsewhere that Leia is indeed referring to Padmé in ROTJ. (I know this has been debated ad nauseam before so I won't delve in any deeper.) But since that statement predates the prequels, I suppose it could mean Leia is now actually talking about Breha instead. And The Complete Star Wars Encyclopedia does indeed refer to "the passing of Queen Breha" in its Royal House of Alderaan entry... But here's hoping we'll see more of her one day!