Just to add that it is not recommended to leave under 16's alone overnight. I think you mentioned night shifts, which in my view present a very different problem from day shifts. I think children's services and the police may take a dim view of children being left alone overnight, and your DIL needs to be aware of this.
So Harry and family are coming back!
Early reading v continental custom - teachers’ thoughts sought
We always had an adult with ours until our younger one was well into his teens (grown up half siblings were a great help) if we couldn't be there.