Frank your Majesty said:
One obvious consequence is that if Leia and Luke are siblings, Vader is also Leia’s father. Vader was directly facing her in Star Wars, being near her for a long time while she’s on the Death Star, and he never sensed anything through the force?
That ties into another point of mine. If they were going to be siblings, I saw no need for them to be full siblings, let alone twins. I would have preferred half-siblings, where she and Luke had the same mother, but different father(s). And she would have been somewhat older than Luke, rather than younger, as the public script of the first film suggested (her being two years younger than Luke). But you’re right, the easiest thing would have been to have not have them be siblings because of Leia and Vader’s previous interactions.