Bill's aunts are called "Faith, Hope and Charity", which Grandfather George claims were named by Grandma after character traits he didn't have. This is a reference to the Bible, specifically 1 Corinthians 13:13: "And now abideth faith, hope, charity, these three; but the greatest of these [is] charity". Bill's mother is called Grace, also a theological virtue.
Eight years earlier, the movie Yanks (1979)'s storyline partially captured the experiences of young English boys during the Second World War, a theme that is elaborated on in this film.
Not only is the string quartet not actually playing the music, but the Mozart serenade in G K.525 (Eine kleine Nachtmusik) is scored for five instruments, not four.
The string quartet of Grace, Faith, Hope and Charity are clearly not actually playing the piece of classical music near the end of the film. Also the Mozart serenade in G K.525 (Eine kleine Nachtmusik) that they are playing is scored for five instruments, not four.