Well, you could just write poly where the two of them are sharing Han without ever actually sleeping with each other, which would have some vaguely incestuous vibes, but wouldn't actually be incest.

Luke/Leia don't meet until adulthood, though, unlike most incest pairings fandom tends to like -- and Lucas himself has them kissing in the first movie. They're the same kind of suprise!incest/unintentional incest that's been a stable of gothic novels and literature in general (Moll Flanders, the collected works of V.C. Andrews) for ages. Luke/Leia actually works within a pre-existing literary tradition of romantic relationships between long-lost siblings who don't know they're related at first. Which doesn't mean it wouldn't be supremely squicky for an awful lot of readers.