Sibley is a city and the county seat of Osceola County, Iowa, United States. The population was 2,860 at the time of the 2020 census. Ha…Sibley is a city and the county seat of Osceola County, Iowa, United States. The population was 2,860 at the time of the 2020 census. Hawkeye Point, the highest point in the State of Iowa, is also nearby.