You need to keep in mind that from '77 to when ROTJ debuted in May of '83, no fans had any inkling that Luke and Leia were related. Even in 1980's ESB during the Cloud City escape scene where Luke is dangling from the antennae reaching out to Leia through the Force (remember she makes them turn the Falcon around and go get him), there was no big "brother sister reveal", so the quick peck on the cheek in ANH and the moderately-intense kiss inside Echo Base on Hoth after Luke is rescued by Han fall entirely within the bounds of "proper" conduct.

I wasn't even alive when ROTJ debuted (born in '85), but aside from the tired Internet jokes and at cons about twincest being wincest and all that, I've never seen any formal outrage or objection directed at that particular loose plot-thread. Certainly not on the level that the Harry Potter books/movies faced on the grounds that JK Rowling was making kinds into Satanists, and the like.