I disagree. They locked him in a room and left to get married, knowing that if anyone had seen Ethan they'd be arrested for his abuse. They refused to allow his father to talk to him. They took extraordinary steps to hide and mutilate his body so that, if found, he couldn't be identified. They waited nearly two days at least to report him "missing", in a plot they concocted to deceive police. NS called Joe claiming Ethan had wandered off. SS attempted to inject an innocent man into the crime, accusing him of helping bury Ethan. They carried on with their life (updating knot and FB pages, shopping, getting married, visiting with family, fixing the playstation, etc.) during this time.
IMO, that is more than enough proof they were in functioning with full mental capacity, fully cognizant of their actions and the consequences of those actions.