Like I said he did tell a half truth by saying that Sarai was his sister. She is his half-sister but he lied by keeping the truth about her being his wife.
The righteous part has more to do with having faith in God. this is what my Bible defines "Righteous"
Abram demonstrated his faith through his actions it was his belief in the Lord that made him right with God.Righteous(ly): Of God, an absolute quality of moral standards; of man, a limited or relative quality, corresponding to a standard of rightness or obligations to God and other persons.
Romans 4:5 "However, to the man who does not work but trusts God who justifies the wicked, his faith is credited as righteousness." (you can also read Romans 4:1-5)
I believe the gist of this is saying that Abram was righteous because of his belief in God. He didn't lie because he was not righteous but because he is still a sinner and he was scared- forgetting Gods promise of protection or at least letting the fear ruled him, and his problems got worse after he lied.
Anyway, Abram was righteous because of his belief in God... which leads me to believe that we are all also righteous because of our belief and faith in God as well.