tuffy said:
But yeah, the whole "Leia as sister" angle is completely stupid on many levels. Since Darth Vader just knows Luke is his son during the fight in Empire, he should know who Leia is while she's stuck in her cell and wouldn't have to find out from Luke by the time Jedi rolls around. Not only that, but it completely wipes out the Luke-Han-Leia love triangle from the first two films as undramatically as possible.

Not really. If you take ROTS as canon, then Vader has been under the impression that he killed his wife before she gave birth. Thus, he's under the impression that he doesn't have any children whatsoever. He wouldn't be on the lookout for his kids, so there's no reason that he'd be scanning random prisoners for genetic similarities to himself. It's not until after the battle of Yavin that he suspects Luke (whose name, presumably, has become famous by this point, and the presence of the name "Skywalker" along with the boy's strength "in the force" is just a bit too much of a coincidence for Darthy to swallow). But even after Vader has become convinced that Luke really is his son, he doesn't suspect for an instant that his wife gave birth to twins. There's no way he possibly could've known that until he ripped it from Luke's mind on DS II.

So, continuity-wise, it works. But I do agree that, from a story-telling point of view, it's pretty stupid and unnecessary.