#21 | Genesis ch.20 - "She is my Sister!"

Why do Sarah and Abraham need to pass themselves off as siblings? Can't they be up-front about their marital status?And in our chapter, as God protects Avraham, Avimelekh complains and says "will you kill the innocent too?" he sounds quite similar to Avrham in ch.18. Why does the Torah choose to put this phraseology into Avimelekh's mouth?