Jamieson, Fausset and Brown on Genesis 20:2

2. Abraham said of Sarah his wife, She is my sister--Fear of the people among whom he was, tempted him to equivocate. His conduct was highly culpable. It was deceit, deliberate and premeditated--there was no sudden pressure upon him--it was the second offense of the kind [see on Ge 12:13]--it was a distrust of God every way surprising, and it was calculated to produce injurious effects on the heathen around. Its mischievous tendency was not long in being developed.

Abimelech (father-king) ... sent and took Sarah--to be one of his wives, in the exercise of a privilege claimed by Eastern sovereigns, already explained (see on Ge 12:15).