Abby and Holly Series, Book 5

The main protagonists, cousins Abby and Holly, find paintings by a young teenager, from the past with a copper key taped to the back of one of her paintings. This teen's remains were found in their attic in Book 4. The girls' search begins to find the source of this key. They hope to use this key to unlock secrets or something valuable from the past. Will this key help uncover another mystery for the cousins? How far will the girls go in their search for truth or will they have to go back to the past? Will they be able to return home without somehow changing history forever?