Why did the original Star Wars: A New Hope posters portray Luke and Leia as a couple?

The posters implied that Luke and Leia were a romantic couple…

Indeed the promotional posters and artwork implied Luke and Leia had some romantic spark going on. The posters themselves were reminiscent of classic Hollywood poster art tropes and film cliches of a brave hero (usually a man) rescuing a “damsel in distress” (damsels are usually women). But typically the “damsel in distress” was not the sister of the hero: It would be some lady so enthralled by the heroic deeds that she fell in love with him.

At this point, obviously George Lucas knew they were siblings. Did he keep it a secret from marketing, or was his intent to deceive the public in order to make the reveal a bigger surprise?

That is because Star Wars has been a proverbial “stone soup” of ideas and never a solid recipe/blueprint written in stone and passed down over the years.

The idea that Luke and Leia had some true romance happening was not accidental and the fact they were brother and sister was still not solidified when Return of the Jedi came around.

And in the case of the first Star Wars in 1977, one of the first trailers for the film has a voice over that clearly states: “The story of a boy a girl and a universe…”

Saying something like that clearly hints at romance… But the key word is “hints”… Having romantic tension in a film between two characters like this is a natural way to add drama to a film. According to Gary Kurtz and others Luke and Leia being siblings was never a part of the original plan. The idea that Luke and Leia suddenly became siblings was some “get this over with” logic that apparently happened to cleanly end the series which was originally planned to contain a larger romance between Luke and Leia.

So in summary:

• Star Wars (1977): Small hints at cute romance and competition between Han and Luke for Leia’s affection but that is about it. Leia kisses Luke on the cheek just before they swung across a chasm in the Death Star. And Han toys with Luke about a potential romantic connection when they have escaped in the Millennium Falcon. So the possibility of romance definitely exists and thus the marketing hinted at that at times.
• The Empire Strikes Back (1980): The film focused far more on character development than the first film—since they were expanding the story of the characters—so the Luke and Leia romance definitely was focused on when Leia plants a full mouth kiss on him in the medical facility on Hoth. The Han and Leia romantic angle was always played up in competition to the Luke/Leia romance and even more in this film where Luke went to some magical planet to train with some woodland troll. Remember, the original theatrical posters had Han and Leia in what is described as a Gone With the Wind pose dead center in the artwork.
• Return of the Jedi (1983): While in this film Luke and Leia are revealed to be siblings the revelation/confirmation of the sibling relationship only came when Ben (Obi-Wan) Kenobi and Luke talked about it on Dagobah after the rescue from Jabba’s palace. And only then was it shared with Leia by Luke in the Ewok village after they are captured by the Ewoks. So at the beginning of the film when Leia is half naked and swinging (yet again) on a rope with Luke from Jabba’s barge, the idea that romance might exist was still a possibility to the viewer and the characters. Thus that aspect still being a focus of the marketing.

Longer answer and explanation of how the “well planned” universe myth of Star Wars is the series’s biggest myth.

When George Lucas—and those involved with Star Wars—endlessly restate the mythology that the whole Star Wars universe was always perfectly mapped out, they are being “truthy”: Yes, George Lucas had sketches of ideas for years of characters and events, but he at no point during the original trilogy have some perfect blueprint in place. The “canon” of the series has always been far more fluid than Lucasfilm (and now Disney/Lucasfilm) will ever admit. For example here are tidbits connected to the first Star Wars film in 1977:

• At one point according to Marcia Lucas (George Lucas’s ex-wife) he toyed with the idea of making Boba Fett and Darth Vader brothers: “George had Boba Fett and Vader being brothers in the prequels, but he thought that would be too hokey.”
• And in the first script for the first Star Wars Obi-Wan Kenobi never died, but just magically disappeared; Marcia Lucas is to thank for that being changed: “I was rewriting, I was struggling with that plot problem when my wife suggested that I kill off Ben, which she thought was a pretty outrageous idea…”
• George Lucas at one point was glib, honest and showed some humility/awareness of his own creative shortcomings as seen in this 1974 interview with Sally Kline in the book George Lucas: Interviews: “But I don’t have a natural talent for writing. When I sit down I bleed on the page, and it’s just awful. Writing doesn’t flow in a creative surge the way other things do.”
• And—more on point to this question—Mark Hamill on that same page confirms Marcia Lucas is the one who insisted the small kiss between Luke and Leia before they swing across the chasm was her idea: “You can see a huge difference in the films that he does now and the films that he did when he was married. I know for a fact that Marcia Lucas was responsible for convincing him to keep that little ‘kiss for luck’ before Carrie Fisher and I swing across the chasm in the first film.”

And in the case of The Empire Strikes Back, here is what Leigh Brackett’s first draft contained. She might have written it, but it was done with the aid of a story treatment from George Lucas and used his ideas. This is a good rundown of ideas that were written but were eventually ditched:

• Han apparently has a powerful stepfather named Ovan Marekal.
• Again on point with this question, there is no romantic tension between Han and Leia. And there is no kiss in the sick bay between Luke and Leia… But… Later on she visits him and the viewers witness Luke and Leia “…in the midst of a tentative, and very tender love scene.”
• Lando Calrissian is a clone.
• The romance between Luke and Leia is strong enough that Darth Vader plans on using her love for him to get to Luke.

And finally the “well structured” work that must have existed by the time Return of the Jedi came around can be revealed as follows; most of this comes from this Wired article unless linked elsewhere:

• The original story had Wookies instead of Ewoks.
• The celebration on Coruscant that was added to the 1997 “Special Edition” was an idea they had floating around, but because they could not come up with a name for the planet they dropped it. The name “Coruscant” did not come from George Lucas by rather author Timothy Zahn who uses it in Heir to The Empire.
• Yoda was never going to show up in Return of the Jedi until George Lucas consulted with child psychologists who explained that Darth Vader’s claim to be Luke’s father needed some independent, third-party confirmation from someone trusted/authoritative like Yoda.
• The whole Return of the Jedi versus Revenge of the Jedi titling issue is not as clear cut as some have made. Some say that because Star Trek: Wrath of Khan was being filmed at the same time under the working title Star Trek: The Vengeance of Khan they decided to change the name to avoid confusion/appearance of competition. Others say it was an effort to mess with bootleggers. But the idea it was changed because “Jedi don’t seek revenge…” logic is not the one solid answer to that.
• At one point of an early draft of the script, Ben (Obi-Wan) Kenobi comes back to life to convince Luke to destroy the Emperor and Vader.
• And in that same script Yoda comes back to life as a flesh and blood creature because of Vader’s death.

Which is all a very, very, very long winded way to say the George Lucas “canon” is not some well planned world that he is painstakingly making sure he get’s as perfect as possible but rather it’s just all one big framework of basic concepts that has been constantly mixed and matched over the years to simply keep an endless story going.

What an early outline of the Star Wars films looked like.

And if you care about the supposedly “true blueprint” of what the Star Wars saga was planned as being prior to 1980, here is an outline that Gary Kurtz—who produced Star Wars and The Empire Strikes Back—has shared in a few places over the years:

• EPISODE 1: Was to focus on the origins of the Jedi Knights and how they are initiated and trained.
• EPISODE 2: Introduction and development of Obi-Wan Kenobi.
• EPISODE 3: Introduction and life of Vader.
• EPISODE 4: There were seven different drafts of the film. At one point, they pursued buying the rights to Hidden Fortress because of the strong similarities. At one point, Luke was a female, Han was Luke’s brother, Luke’s father was the one in prison (interesting point for some debates) and the film featured 40 Wookies.
• EPISODE 5: Once written, the screenplay of Empire is almost exactly what is seen on screen. The only cut scenes were those involving wampas in the rebel base (cut because of time and unsolved technical glitches) and about two minutes of Luke/Yoda Jedi training with no real dialog.
• EPISODE 6: Leia was to be elected “Queen of her people” leaving her isolated. Han was to die. Luke confronted Vader and went on with his life alone. Leia was not to be Luke’s sister.
• EPISODE 7: Third trilogy was to focus on Luke’s life as a Jedi, with very few details planned out.
• EPISODE 8: Luke’s sister (not Leia) appears from another part of the galaxy.
• EPISODE 9: First appearance of the Emperor.

The poster is a relic of incomplete storywriting.

Luke and Leia weren't originally going to be siblings. The poster was made before that time, and featured a fairly standard fantasy trope -- the damsel clinging to her savior.

Axelrod has the correct answer.

Another angle, however, is that in Star Wars: A New Hope, they essentially were somewhat romantically connected. In fact, that was pretty much a sub-plot… Han and Luke competing for the the attention of the princess.

And that’s fine, given that Leia and Luke—and the audience—weren’t aware that they were siblings at that time.