Too many flaws.
I know the major complaint about this movie are the number of flaws in the story but I thought I'd give the film a chance anyway. Surprisingly I didn't HATE it, whilst I didn't really like the kids too much the fake grandparents were decent to watch and they carried the movie imo. Apart from that, it is really difficult to thoroughly enjoy the film because of the number of plot holes that arise.
As a mother, knowing the type of people that exist in this world and what they are capable of, would you really send your kids off to your parents place whom you haven't seen for a long time without phoning them or contacting them in any way to make arrangements for the trip? This is the biggest plot hole as has been mentioned by a lot of people on here but there are a few more.
Evidently this couple were in the mental hospital and since the doctor from the hospital comes around to check on the real grandparents that haven't turned up for their regular volunteering slot, wouldn't alarm bells be ringing in his head? Not only do two of your regular volunteers not turn up but two patients from your hospital go missing around the same time, wouldn't that warrant a call to the police to check things out? I mean as a doctor, he knew how disturbed the fake grandparents were and one of them had killed before, it wouldn't be a huge stretch to think they had perhaps killed again would it?
Then you have the ex addict turning up and instead of her getting in her car and heading straight to the police to tell them what was going on, upon meeting the couple she seemingly allows herself to be coaxed inside or around the back where she is killed. Again, she must have realised something fishy was going on and likely knew the couple were deranged and unstable but she chooses to follow them as opposed to getting the hell out of dodge and sending help?
When the mother finally sees her so called parents and realises what is going on she calls the local police department who don't answer, fine. But if they didn't answer, wouldn't you call 911 instead? Surely someone would have been able to radio through to the local police to tell them to get out to the house asap?
And yeah, if you arrive at your grandparents house to be told to stay in your rooms after 9.30 and never go to the basement are two teens really going to listen to that?