He belongs to a cadet branch of the former royal dynasty of France (and current dynasty of Spain), the House of Bourbon, which ruled the independent Duchy of Parma in Italy until 1859, and reigns (patrilineally) today in Luxembourg.
Sixtus was born in Pau, Pyrénées-Atlantiques during the World War II Vichy regime of France, the second son of Prince Xavier of Bourbon-Parma (then Prince Regent, later Carlist pretender to the throne of Spain, later titular Duke of Parma) and his wife Madeleine de Bourbon-Busset.