The fact that they're both called skywalker would be a bit of a clue though even if you could find a way to keep it secret till Empire. In fact, why the hell did the Lars/obi wan not change Luke's name for his own protection. In fact, why the hell didn't Obi-wan change his name - calling yourself Ben Kenobi is hardly a foolproof alias - luke put 2 and 2 together quickly enough. And hiding baby luke with Darth Vader's step brother (only living relatives in fact) wasn't too smart either.