You'll notice that the parent Reimu's foot (feet?) has been burnt and its mouth sewn shut. The parent Marisa appears to have been crushed flat. Why no one leaves the cave is a Fridge Logic question.
My real question is whether this is Mister's doing, or is it that Reimu that shows-up later in the story?
How the hell did Mystery-reimu manage to burn mother's bottom and sew her mouth shut without, you know, hands? This is anon's handywork. Fridge Logic has no place within that explanation. And who is that reimu, anyway?