"Robert I (c. 1035–1093), known as Robert the Frisian, was count of Flanders from 1071 to his death in 1093. He usurped the countship after defeating his nephew Arnulf III and his allies, Robert´s sister Matilda and King Philip I of France. He made peace with Philip, who became his stepson-in-law, but remained hostile to his sister and brother-in-law, King William the Conqueror of England." - (en.wikipedia.org 08.10.2020)