The books they are reading
Throughout the movie, characters are shown with Penguin Paperbacks in hand. I first noticed it because the brother, Paul, was reading Robert Graves' I Claudius, a book about a plotting and warring royal family in Ancient Rome. Then I saw that sister Blanche had Aldous Huxley's Antic Hay at the breakfast table, which was, incidentally, quoted in Brideshead Revisited, another glimpse into a declining family legacy.
I just wanted to send a shout out to the clever property master and set designer. Nice touch.