william was the second son of count william i of geneva (died 1195) and younger brother of count humbert i of geneva. when humbert died in 1225, william seized the county and expelled humbert's sons, his nephews, peter and ebal, who eventually found protection under peter "le petit charlemagne", who had brought them with him to england by 1244. that year peter, the elder, married a rich english heiress, mathilda de lacy. peter "le petit charlemagne" was in fact william's nephew, a son of his sister margaret.