I knew the Adam Brody (brother) would turn on the family and the groom would turn on Grace by the end
The Aunt pretty much laid it out when she said the groom was the chosen one and once the mother told him he wouldn't kill her that confirmed it.
Grace inherits the family fortune right, she's the only family member alive even though this shyt would be tied up in court forever
Although all she has to do is point the cops to the goat barn where all the sacrifices and bodies are