When Episode 4 was filmed, did Lucas know Luke and Leia were brother and sister?
To quote from my answer to Leia's musical theme:
Even an early Leigh Brackett's 1978 draft of "The Empire Strikes Back" didn't have that relationship established yet - it was introduced later (Source: http://geeksofdoom.com/2010/05/15/early-draft-of-empire-strikes-back-reveals-alternate-star-wars-universe/ ).
Another related evidence for Luke and Leia not yet being related was 1978 pre-ESB book that was a sequel to "ANH" - Alan Dean Foster's "Splinter of the Mind's Eye". It develops the theme of romantic interest between the two (presumably, in part, since Foster and Lucas didn't know if ANH would be a big hit and if Harrison Ford would be interested in the sequel should the sequel be based on "Splinter").
According to Lucas, yes.
However, wookieepedia has this to say on the relationship in Splinter of the Minds Eye (the first Star Wars EU book):
At the time it was written, the familial relationship between Luke, Leia, and Darth Vader had yet to be revealed. Therefore, some inconsistencies are evident and unavoidable. Most noticeably the sexual tension depicted between Luke and Leia in this book, which in retrospect would be considered inappropriate in most countries. (Although Lucas claimed to have had the Star Wars saga mapped out even at this early stage, the fact that he allowed this plot element to remain in Foster's novel has been cited as evidence that he hadn't yet decided on the characters' true backstory.) However, Lucas has stated before, that this tension was on purpose, to show that the two had feelings for one another, but that they did not know exactly what type of feelings.
Some background on Splinter:
Splinter of the Mind's Eye was the first-ever Expanded Universe novel to be written and published. According to an interview with its author, Alan Dean Foster, in Empire magazine, the novel had been written to be filmed as a low budget sequel as a fallback plan in case Star Wars hadn't been a huge success. This accounts for why the book takes place almost entirely on a fog-shrouded planet. Additionally, Harrison Ford was not signed for the sequel as of the writing of the book, which is why Han Solo does not appear in it. Although George Lucas is credited as the author of Star Wars: From the Adventures of Luke Skywalker, the novelization of Star Wars Episode IV: A New Hope, was in fact written by Alan Dean Foster. Foster's early involvement in the Star Wars universe gave him the opportunity to write this book. Though Foster was granted a great amount of leniency in developing the story, one requirement was that a lot of props from Star Wars Episode IV: A New Hope could be reused when filming the new film. According to Foster, Lucas' only request upon inspecting the manuscript was the removal of a dogfight in space undertaken by Luke and Leia before they crash-land on Mimban. Presumably, this sequence would have cost too much money to create.
I don't agree [with the other answers.] It is established in Empire Strikes Back that they are brother and sister. Near the end, when Luke is hanging onto the city's [underside antenna], he begs for Ben and gets no answer. Then he says "Leia, hear me, Leia," and she hears him through the power of the force, establishing her as the other one.