Augusta Victoria of Schleswig-Holstein was a minor princess distantly related to Queen Victoria, and married a distant relative, the last Kaiser of Germany.
Augusta of Saxe-Weimar-Eisenach: The First German Empress
Augusta of Saxe-Weimar-Einsach was just another German princess who became Queen of Prussia. But after the unification of Germany, she became an Empress.
Alexandra of Bavaria: The Glass Princess
Princess Alexandra Amalie of Bavaria (1826 – 1875) is famous for believing that she swallowed a glass piano as a child. There's even a play about her, The Glass Piano.
Sibylle of Cleves: Henry VIII’s Sister-in-Law
Sibylle of Cleves was a sister-in-law of Henry VIII who is all but forgotten. She was the eldest daughter, a mother of four Johns, and a brave defender of Wittenberg.
Helene in Bavaria: The Sidelined Sister
One tends to remember an Empress of Austria more than a Princess of Thurn and Taxis. But Helene in Bavaria led a much happier life away from the imperial throne.