To go inside the old cabin at chapter 1, you are told by a childhood Amelia that you had it locked with a key. Find the key and the doors will open. To find it look in the old tree outside the cabin.
This key represented a lot to you and too Amelia. To you it was something you and Amelia could share. It was the doorway to a fun childhood of adventure. To Amelia it was a way to rebel against her dad; a place that even he couldn't reach her.