Who is XXX's sister?
Frederick the Simple
``Frederick the Simple'' is often confused with an earlier Sicilian monarch, Frederick II, who chose to call himself ``Frederick III'', even though he was actually only the second King Frederick to occupy the Sicilian throne, as also this Frederick has been dubbed by later generations of genealogists and historians as Frederick III. Frederick III was born in Catania, and succeeded to his brother Louis in 1355 under the regency of his sister, Euphemia of Sicily.
Euphemia of Sicily