It trolled John Williams too. At a concert of his he said that he had no idea they were siblings, and was completely embarrassed about his theme for them after the reveal because the theme reached a very loud climax, which didn't quite make sense between siblings![]()
It's possible Luke and Leia were made siblings when Lucas shelved the original story for Return of The Jedi. In it Han was going to be killed off and Leia was going to leave Luke and govern what was left of her people. When Lucas decided not to go down that route he may have made Luke and Leia siblings as a tidy way to resolve the Luke-Leia-Han triangle.