Agreed. The real answer was just like you said, Lucas’ relaxed views on continuity and if it is really that important. There’s definitely ways to explain it though I think. I did use to feel that it was one of the weaker elements of the OT, as far as it having very little set up and feeling tacked on. But over time I’ve realized the very PG incest vibes give Luke another similarity to King Arthur, who in some iterations of his legend had unknowingly slept with his own sister, resulting in Mordred, who would go on to betray Arthur and destroy Camelot.
Not really prequel related I suppose. Though you could argue the prequels parallel Arthurian legend as far as the Republic being like Ancient Rome which feel in the time before Arthur, and his father Uther Pendragon being analogous to Anakin in some ways. So the prequels continue that tradition of mirroring real myths that the OT started.
I don’t think these parallels were intentional but they do make me enjoy them more. Repeating ideas, even if it was unknowingly.