Luke, Obi-Wan, Vader and Anakin were all connected from the very start. Making Vader Luke’s father in ESB was a nice way to break with the cliché of “hero avanges his father’s death”. But introducing Leia as Lukes sister didn’t add anything to the plot. In Star Wars, she’s a diplomat and member of a royal family. That’s enough for the plot, we don’t really need to know more. By making her Luke’s sister, you need to explore her background, as it creates more questions than it answers.