God may not approve of lying, but he seems okay with lying by omission. Thank God for loopholes!
Have you ever been at a family gathering and someone begins telling a story, only to be interrupted halfway through by another family member saying that they had the details wrong? Well, that's exactly what some biblical scholars believed happened here with Genesis 20. We saw two versions of creation, two versions of Noah's ark, and now we're seeing two versions of Abraham's journey.
This probably came about because the compilers of Genesis didn't vote on the correct version of their stories, they just included both of them. This yields two similar stories with different details (there will be a third by the end of Genesis!). To a rational person, it all seems pretty obvious, but biblical literalists are forced to make up all sorts of wild explanations and excuses to try and get these repetitions to make sense.
We also learn a rather disturbing fact, namely Sarah is indeed Abraham's sister! Specifically, his half-sister; they both had the same father. I guess Abraham decided to go all Luke Skywalker on
Leia Sarah. Strange that there wasn't any mention of this incestuous relationship in the first telling of the story.
Another thing that seems to be left out of the first story is the fact that, here in Genesis 20:13, Abraham actually pulls the time-honored line, "You'd do it if you loved me," in order to convince Sarah to lie. The NIV actually translates the verse to, "This is how you can show your love to me, … say of me, 'he is my brother.' " Awesome!