The Totally Batsh*t, 100 Percent Insane Twist
The real Brahms has been living between the walls of the house this whole time. He's been spying on Greta, eavesdropping on conversations, and making his way around through secret passages, crawl spaces, and other nooks and crannies.
That's how the doll moves. That's how he knows if you're breaking his rules. The leftovers in the freezer are how he manages to survive.
Yes, his parents knew. He was more or less holding them hostage. That's why they hired Greta; she was meant to be a "gift" so that Brahms would let them go. The letter they sent is to let Brahms know that they won't be returning, and Greta is his for the keeping.
So, what happens? Brahms immediately kills Cole, then goes after Malcolm. Greta manages to escape, leaving Malcolm behind. She suddenly has a change of heart, though, and goes back for him.
Knowing Brahms must stick to his rules, Greta commands him — the real, 28-year-old human man — to go to bed. After a moment of confusion, Brahms agrees. He demands his goodnight kiss, and although Greta obliges, she also stabs him multiple times in the stomach. Seizing her opportunity, she grabs Malcolm, and they make their escape.