The thing is, that was part Lucas wanting to put the saga to bed (until he realised he couldn't do anything else), and partly a result of him putting the saga to bed, because everything got condensed down pretty severely. It was pretty clear Luke was meant to have a sister, but the reveal of her being his sister kind-of flies in the face of the teasing moments Lucas had in Hope and Empire, where he kind-of suggested things but we eventually got Han/Leia.

Which is why we get the nonsense 'I've always known' dialogue in Jedi, which makes about as much sense as Ben's 'points of view' monologue.