I think Carol, as a religious woman, kind of took it upon herself to raise and mold her daughters into the Roaring Knight and the Monster girl. Kind of forcing the prophesy into being. I think Dess was raised by Carol to accept her impending death, so when Carol had her killed once she was old and strong enough, she'd fufill her role of the prophecy in the dark world.
I think the Guitar has Dess dusted over and in it, which in the Dark world created the Roaring Knight. And she doesn't want anyone to touch Dess's old room to not stretch Dess's essence too thin or she'll dissapear.
I think a later chapter will deal with Noelle coming to terms with the fact that her sister really is dead and stand up to her mother. Carol thought she was doing the tough but righteous thing to do, but Noelle will make her confront the fact she raised her own first born daughter as a lamb to the slaughter. And the fact that the dark world isn't like, real. Carol will have to confront that she really did it all for nothing.