Since this has already been bumped. . . .
I would disregard Leia as Luke’s sister – not because of twincest*, but because it’s universe-shrinking bullshit. Alas, too much of the post-ROTJ EU I accept as canon predicably runs with the revelation, so, as it is with the Death Star II, I accept it but only with much wailing and gnashing of teeth.
*Consenting adults can do what they want.