Or HaChaim on Genesis 20:2/אור החיים על בראשית כ׳:ב׳

1 ויאמר אבוהם אל שרה. Abraham said about Sarah, etc. This time Abraham did not ask Sarah to be the first to announce that she was Abraham's sister as he had done when they travelled to Egypt. Then he had been motivated by the ugliness of the Egyptians and the contrast with Sarah's white skin which aroused the lust of the Egyptians. The people in Geror did not look different from Abraham and Sarah. Also, their general sexual mores were superior to that of the Egyptians. Avimelech himself felt justified in challenging Abraham's deception if it were based on such suspicions. Pharaoh had not dared to ask such a question. This is also the reason that Avimelech agreed that Abraham reside in his country, something the King of Egypt could not afford to do.