Matthew Poole and continuators on Genesis 26:8

8 And it came to pass, when he had been there a long time, that Abimelech King of the Philistines looked out of a window, and saw, and behold, Isaac was sporting with Rebekah his wife .

Using more free and familiar carriage then became a Brother and Sister, but such as was allowable between Husband and Wife: See Deut. 24. 5. Prov. 5. 18, 19. But that this was not the conjugal act may easily be gathered from the circumstances of the time, and place, which was open to Abimelechs view, and therefore that was not consistent either with Isaacs modesty, or with his prudence, because he would not have her thought to be his Wife.