There’s also the fact that the storyline for the Sequel Trilogy involved Luke meeting & falling in love with his long-lost sister Nellith (culminating in a movie where their son goes evil, Mordred-style).
Nellith’s character arc was the inspiration for Mara Jade in Timothy Zahn’s Thrawn trilogy. Zahn tipped the hat to her origins as Nellith by putting the Branstock tree from Wagner’s Die Walkure in Talon Karrde’s base in Heir to the Empire (which he mentions in the HttE annotated author’s notes).