This passage occurs in the Narrative Lectionary Year 1 (Most recently Feb 22, 2015).
εθνικος (pagan, gentile, literally "ethnic"; 18:17) Jesus suggests we
treat Christians who have greatly sinned against us as gentiles.
Interestingly, Jesus final words in the Gospel of Matthew instruct us to
preach to the gentiles (all the nations of the world; same root word)
and earlier Jesus reminds us to love our enemies, because even the
gentiles to this. Jesus is not giving us permission to be rude and
dismissive to our brothers and sisters in Jesus, even those whom we are
When you break down the Greek, you realize that Peter does not wonder "if" his brother will sin, but he wants to know what to do "when" his brother will sin.