It's academic anyway.
Outside the EU we don't know how Vader found out who Luke is.
I assume that there was an investigation into who it was who destroyed the Death Star, the name came up and it made sense when Vader took into account his own perception of a pilot unusually strong in the Force, it makes sense he was told by spies.
Luke didn't just figure out Vader was his father, Vader told him and it was confirmed by Yoda and Ben later.
To have Luke just figure out Leia is his sister with very little if any forewarning or observed evidence, based solely on some half whispered dying words about "another Skywalker" is pushing it a bit.
How was he even supposed to leap to sister from that information?
Uncle, mother, grandfather, brother, cousin...and then to a name?
Even Lucas wouldn't make that much of a leap.