Good but...

I wish there were more survivors. They should have just started listening to Erin. I also didn't like that the cop at the end died. Should have had a few of them who survived and the cops arrived and they were all safe.

Also none of the family members should have been in on it. Would have been better if they were just attacked by outsiders. That twist has been used to death.

Aside from these things, this would have been not another cliche slasher.