According to her Wikipedia article, Sybille was the oldest daughter of Johann III der Friedfertige, Herzog von Jülich-Kleve-Berg and Maria of Jülich-Berg. She had two sisters, Anne and Amalia. Anne was briefly married to English King Henry VIII - she escaped that marriage with her head still attached. There appears to be just one image that may be of either her sister Amalia - or Anne.
Cleves was a Duchy of the Holy Roman Empire with its capital at the city of Kleve (Cleves) close to the German side of the present day Dutch-German border. It is located in the state of Nordrhein-Westfalen. It was called Cleveland in old English. Kleve was flattened in 1945 by the Royal Air Force as a preliminary to the Anglo-Canadian crossing of the Rhine (the Americans were similarly engaged further south).