The Field Guide discussion

After their parents' divorce, the Grace kids - twins Jared and Simon and their older sister Mallory - move with their mom to the decaying old mansion belonging to their ancient great-aunt Lucinda, who has been sent off to an asylum. But before they can even unpack, Jared begins to notice that they're not alone in the house.

Can you explain who they are and what is happening?Lucinda's Secret