Here's a little addendum concerning the later drafts:
I think I figured out why Ralph McQuarrie drew Luke and Leia as looking so identical in his SW 1977 concept art from the third draft onward. And it's not because Lucas already imagined Luke and Leia as brother and sister.
First, go back to this famous McQuarrie painting from the SW second draft, from the point in time where Luke was a girl:
Look how McQuarrie has painted both girl-Luke and Han Solo. Specifically, they're both blond. The only difference is that Han has a red beard.
Now we know that Han is definitely not Luke's brother in this version; that role belongs to Deak Starkiller. So what's going on?
I suspect Lucas instructed McQuarrie to draw the two principal leads of the film as looking almost exactly alike, to show that they were soul mates. Like, and yet unlike (as indicated by Han's beard). Girl-Luke is the young innocent, the naive farm girl heading out into the wider world for the first time. Her love interest, Han Solo, is an experienced smuggler, a confident and self-assured man who knows the galaxy. Yet each of them has something of the other inside as well; yin and yang. Luke will become a bold Jedi warrior, whereas Han runs away from the Imperial might of the Death Star. Thus they are perfectly matched, and a natural romantic pair.
Ralph McQuarrie presumably transferred this symbolism intact to his later concept art based on the third draft. There the two lovers assumed the form of the naive farmboy, Luke Starkiller, and the confident leader of soldiers, Princess Leia. But they were still intended to end up together romantically, so they were drawn to look very much alike--like two halves of one whole being.
McQuarrie seems to have imagined both Luke and Leia as blonde by default, and both with short hair. However, when he drew the two of them together, he would usually alter the hair color of one of them slightly, making it light brown instead. This has the effect of making the two characters look not quite alike--just like Han Solo's red beard did in his character design from the second draft. (Alternatively, in his painting of the medal ceremony at the end of the film, McQuarrie gave Leia long blonde hair to contrast with Luke's shorter bowl cut.)