Originally Posted by Dark Raven
They didn't know they were brother and sister at the time, having grown up separately. Once they found out they didn't keep it up. Was this fact even known at the time of A New Hope? Or was it something Lucas changed later on by the time of TESB?
may not, tho, it's still kinda messed up that he was like instantly attracted to her
and you know who was suppose to know, Obi-Wan, he's giving Luke all this advice (as a force ghost) he couldn't throw out a "stop trying to kiss your sister"
but, yes it has been said that them being twins was kind've a last minute thing they added to TESB