Teen/YA/Horror Fantasy book about Girl who moves to France and Explores world war bunkers with a boy she meets

Pretty much title. All I can remember is that the girl moves to France with parents and her little brother who doesn't speak and she meets a boy (Possibly called jeanne or some variation) who lives with his Grandpa, they go exploring a World War Bunker that is haunted by ghosts or ghouls. They end up releasing these spirts and one might have been of a girl that the boys Grandpa dated (she may have been German, no idea), the little brother ends up on the train tracks and the boys races past the girl to save the little brother and they trap the ghosts/ghouls by breaking the door with a grenade or explosion