Derived sites are part of any Site object of the content schema by means of the field derivedSites (see Section 5.5, “Localized Content Management” in Blueprint Developer Manual for details).

query DerivedSitesQuery($id: String!) {
  content {
    site(siteId: $id) {
      name
      id
      locale
      derivedSites {
        name
        id
        locale
      }
    }
  }
}