If you notice when Abraham (a prophet of God) who no one is to do no harm to said to Sarah, This is thy kindness which thou shalt shew unto me; at every place whither we shall come, say of me, He is my brother (not your husband). It was for the sake of harm coming to Abraham. He told the truth (but not the whole truth) Isaac did the same with Rebekah In Gen 26:9 (lest I die, he said)
And in saving Abraham alive (in doing so) was the very kindness he asked Sarah to show. Same with Rahab (as she said) in Josh 2:12 Now therefore, I pray you, swear unto me by the LORD, since I have shewed you kindness, that ye will also shew kindness unto my father's house, and give me a true token (saving her house alive) also.
They would have killed Abraham for Sarah's sake (as her husband) but only knowing he was her brother, they would treat him well (and spare him) if he was only a brother. God (personally) revealed the full truth to the other and instilled a fear of Himself to them (if she was not returned to Abraham). So twofold God had kept the other from sinning against him (because of what Abraham and Saran both said in relationship to themselves) as brother and sister (rather then husband and wife) which is the full truth. But I also see Abraham being spared from being killed (as a mercy and kindness) just as it says, touch not mine anointed and do my prophets no harm too.
Gen 20:7 Now therefore restore the man his wife; for he is a prophet, and he shall pray for thee, and thou shalt live: and if thou restore her not, know thou that thou shalt surely die, thou, and all that are thine.
Abraham revealed half the truth, God revealed the other half concerning the fact that she was his wife, God also instilling a fear in him now (in being fully aware of this fact) and that he should restore her (and what judgment would be rendered unto him if he did not). God establishing (to him) that Abraham was a prophet of God. So I sorta see him being spare the killing of a prophet of God (firstly). Going with one half of the truth (my sister) and God revealing the other half in that scenario.