Like weather Han shot first, George Lucas hasn't been very transparent about whether he really planned to have Luke and Leia kiss, and then turn out to be brother and sister.
A theory that I haven't seen explored anywhere, is the idea of genetic sexual attraction. It has occurred in brothers and sister before, such as in this tragic case of twins.
Genetic Sexual Attraction is the phenomena that people who are closely genetically related can be attracted to each other if the first time they meet is in adulthood.
The fact that there were more scenes means that the most likely explanation remains that the reveal in Return of the Jedi, was a later edition to the story line, so wasn't known at the time of Empire Strikes back