Luke and Leia were established as twins in Return of the Jedi, but there is more to the backstory of their link than meets the eye. The characters' relationship may be a Star Wars staple, but their connection wasn't always planned. A New Hope set up the idea of them being attracted to each other before The Empire Strikes Back planted the seeds for their familial connection, which Episode VI ultimately confirmed. The fact that making Leia and Luke related wasn't originally planned is well-known, but the retcon's background is more obscure. However, 1997's Star Wars: The Annotated Screenplays offered insight into how the twist influenced Episode VI.
In the book, George Lucas revealed that while writing Return of the Jedi's Throne Room scene, he struggled to come up with something that would enrage Luke enough to lose his self-control while fighting Darth Vader. The creative struggle came up concurrently with the concept of Leia being Luke's sister gaining traction. This made Lucas realize that the prospect of Vader turning his sister to the dark side would make Luke lash out the way he did. Thus, the nascent concept of Luke and Leia being siblings provided a major emotional jolt for Vader and Skywalker’s climactic fight.
Luke's Relationship With Leia Was Important For The Throne Room Scene
While Leia being Luke's emotional drive during his duel materialized relatively late in the creative process, it worked as a perfect catalyst for her brother's catharsis. Luke went to Darth Vader and Palpatine in Return of the Jedi to save his father. And, as he stated in the Throne Room scene, he had no intention of battling the fallen Skywalker. With that, as mentioned, the sequence needed something that could believably make Luke take the violent turn he experienced, which would, in turn, heighten the moment's emotional gravitas. This is where Leia proved vital, as Lucas detailed in The Annotated Screenplays:
"In the end, I had a problem in the fight between Luke and his father of why [Luke] makes the final turn to the bad side of the Force and tries to kill his father. [Director Richard Marquand] was trying to block out the fight between Luke and Vader, and we got down to that point underneath the throne room there, and he said, 'You know, the script sort of says that Vader says something that upsets Luke,' or something vague like that. [...] And we didn't have that actual moment that we needed where you got the sense that Luke is hiding. He's not going to fight him. [...] He'd rather die first, and then, something turns him around and makes him fight. [...] And in the process of evolving the script and evolving the importance of Leia as the sister, it was sitting right there in front of my face, and it became obvious that turning her to the dark side would be the thing that would set Luke off again."
Luke showed glimpses of his brashness and anger throughout Episodes IV and V, but it wasn't until the Throne Room battle that he unleashed his wrath. He lunged at Vader, his attacks no longer precise or thought-out. The hero's rage showed as he wildly swung his lightsaber, attempting to kill the person he was so vehemently trying to save. The sequence made it plausible for Luke Skywalker to succumb to the dark side, and ultimately showed his virtue when he resisted his violent urges. It was a pivotal moment for the character, which likely wouldn't have been the same without Leia as its indirect catalyst.
Luke & Leia Being Twins Fit With Star Wars Lore
Despite its importance in the Star Wars mythos, Luke and Leia's connection has often been seen as a contradiction to A New Hope and The Empire Strikes Back due to the movies’ hints of Luke being attracted to Leia, with the biggest one being their infamous kiss in Episode V. However, while it’s true that their familial link came about gradually over the development process of Star Wars’ original trilogy, the twist still fits the lore established in the first two movies. Firstly, the heroes being unaware of their connection could be justified by them not being proficient enough in the Force when they met to realize they were twins.
Secondly, their family connection was (perhaps unwittingly) heavily hinted at during Episode V's third act. Following his fight with Vader, Luke dangled from below Cloud City and called out to Leia through the Force, after which she sensed and promptly found him. While the scene may not have been intended to hint at their sibling relationship, it ended up being a very fortunate case of retroactive continuity for it. Therefore, while Luke and Leia being twins wasn't planned, and though there are things that seemingly contradict the reveal in Return of the Jedi, the character's backgrounds and Episode V’s hint allow for their connection to fit into Star Wars' lore.