ways to trap a Diana
What about battery operated lights with motion sensors? Does she create motion? Presumably, she flicks the switches to the lights in the house. But if each room were equipped with motion sensor lights (outdoor security lights), she couldn't turn off the lights, because any movement she made would turn on the motion-activated lights. It would make her paranoid about entering rooms.
Also, put UV lights in every room the house (along with motion activated floodlights).
If Rebecca and co wear to have taken their mom outside during the day and tied hr to a chair and tried to kill her, Diana would HAVE to try to save Sophie's life, but she'd have to step out into the bright sunshine to do it. You'd have to use teh mom as bait.
Not sure any of that would destroy Diana. Just punish her.