Is your niece old enough to be home alone? Could she smuggle the dog out of the house and say it bolted when she opened the door? She seems to care enough to help somehow.

When I worked at a kennel, I met dogs who didn't know how to play. You could always tell which ones just plain didn't like to play and which ones didn't know what a toy was. It was always heartbreaking when they didn't know what a toy was.