OBI-WAN: That boy is our last hope.
YODA: No. There is another.
See, what this tells me is - in Episode III, Mrs. Vader should have given birth to Luke, Luke should have been passed on to Ben, and Ben should have been ushered off to the desert - therefore, Mrs. Vader's final fate (and Leia's birth) would remain a mystery to the unfortunate viewers who may someday watch Star Wars sequentially, from the Phantom Menace through to Return of the Jedi.
"And that is why your sister remains safely anonymous."