You're right that Obi-Wan's story about Luke's father being betrayed hints at more dark details. It still wasn't what Lucas originally thought of, by all accounts I know of. Similarly, Luke reaching out to Leia seems consistent with them being siblings, but isn't necessarily indicated except by juicy-plot-twist-invention-logic, as Empire already had Vader intentionally torturing Luke's friends because he expected Luke to be attuned to them so that the torture would send a psychic signal that would lure him there.