This made NO sense to me (SPOILERS)
1) How did they know anything about Anna? How did they know that the parents had never seen Anna or that Anna was supposed to babysit instead of Maggie? And whose house was Emelie sitting in front of? I assume it was Anna's? But then her mom looks out the window when she says, "Bye, Mom." The mother didn't think it was odd that there was some strange girl sitting on her front step yelling, "Bye, Mom?" Not to mention that, at this point, her teenage daughter is missing.
2) Why wouldn't you just kidnap a baby? A baby that didn't have other siblings that could foil your plan? The kid playing Christopher was adorable, but once he realized he was being kidnapped, he probably wouldn't have been as much fun. He would have started missing his parents and screaming bloody murder. The child that died was just a baby, so why kidnap an older child?
3) In 2016, who keeps a VHS tape of them having sex with their mistress? And why was it so easy to find? I assume that's something he was hiding from his wife, but Emelie was in the house for an hour and found it right away.
This isn't a question, but just a thought...I thought the "twist" was going to be that the father had molested her when she was a younger girl (like "Martyrs") and she wanted to get revenge by torturing his children. And then the video tape could have been evidence of what he did. (Plus it would explain why it was a video tape, because it was old.) And then she drugs the kids so they pass out and the parents come home and don't know anything's wrong. Mom passes out because she's had too much to drink and Emelie seduces the dad, which he's into at first, until he realizes who she is. Boom. Way better movie. Maybe a bit predictable, but at least it would make sense.