Caught up on ep 9 ready for the finale, I thought it looked like Piper in the costume that knocked out the Sheriff. I still think she is the daughter of Brandon James and that she wants to expose the Sheriff as the real killer from back in the day.
Emma needs to hope Piper is lying about Kieran or she did the deed with her half brother.
My final guess is that The current day murdering duo, perhaps working in conjunction with Piper, are Brooke and Noah. although don't be shocked if we don't get a definitive answer in tonight's finale.