Trying to find the name of a YA fiction book set during the war?

Im trying to find a book that i read years ago. From what i remember its about a young girl, possibly sent to america during the war, where she lives with an american family. Her name is possibly Frances, which the american family shortens to Franny or Fanny. Her brother goes to war on a ship she discovers that he is lost at sea. The young girl eventually befriends the daughter of the family she lives with and i think at the end even falls for the son, who initially hated her and the war, but ended up joining the forces at the very end.