Puggo - Jar Jar's Yoda said:

When ESB was produced in 1980, Obi-Wan didn't even know Leia was Luke's sister. I don't even think George knew that.

There was nothing to "know". It's one of the sloppiest, half-baked, last-minute, tacked-on, loose-end burning retcons in genre history.

When they had Obi-Wan deliver the twins to Ben's farm in ROTS, nobody at Lucasfilm stopped and said: "oops, loss of continuity, we can't do it that way".

They may have said that to themselves, but being the good Yes-Men they are, they kept their mouths shut.