I always assumed that Luke's parents were married, giving him a nice Christian/non-bastard birth just like me.
If, instead, Anakin is on some backwater planet in the middle of the Clone Wars and he impregnates some local girl... he wouldn't necessarily know that he had a kid.
And, by that token, does Leia have to be his twin? Could she be the daughter of another woman?
How would you react to this if it was in the prequels? Does it change Luke's character much?