Missed opportunity with mother/diana storyline
Horror is about being freaked out, and the freakiest scares come from the ones we don't expect. Like the mother saying "Did we wake you?" - This could have gone further, not just a friendship, but a romance.
"We're going to be a family now."
"I don't want her!"
"She's going to be your mum too so get used to it!"
And then you could have a scene where the boy is being soothed to sleep and he closes his eyes and feels Diana's claws on his face.
"Don't freak out honey, she just wants to be mom too."
Eventually the boy accepts the new reality and it is up to the sister to save him.