Goodgulf:
smeple: I'm more of a Star Trek fan than a Star Wars one, but didn't it turn out that Luke and Princess Leia were brother and sister?
Yes, but they didn't know they brother and sister until the third movie. There's a nice passionate "kiss for luck" in the first movie and up until the end Hans thought Luke and Leia were going to be lovers.
Yes, but OTOH in "The Empire Strikes Back," it's quite obvious (although she's in denial about it for most of the picture) that Leia is strongly attracted to Han Solo rather than Luke. She finally admits it to Han just before he's about to be carbonite-frozen for delivery to Jabba the Hutt:
Leia: "I love you."
Han: "I know..." (This would otherwise seem a bit arrogant, except that he's been trying to get the princess to confront her feelings about him throughout the movie.)
Luke earlier does get another sweet kiss from his unknown-to-him sister, while he's in the infirmiry on Hoth, but that's merely an attempt by Leia to 'prove' that she's not attracted to Han.
When Leia, disguised as a bounty hunter, frees Han from the frozen carbonite near the beginning of "The Return of the Jedi," he temporarily can't see and her voice is altered by the full-face helmet she's wearing. When he asks her, "Who are you?", she slips off the mask and replies, "Someone who loves you."
Since Luke is the primary hero of the trilogy, it's 'bad form' for him to lose the girl to even a friendly rival, but his sudden discovery that Leia is his twin sister (something foreshadowed in "The Empire Strikes Back") thus eliminates him from any competition for her romantic affections.
Of course, I do wonder what the princess ends up having to do for/to/with Jabba, while attired in a 'harem slave' outfit--she seems pretty enthusiastic about later strangling him to death when the opportunity arises...

--C.K.