Thread:Astarothfan/@comment-27083298-20170611064602/@comment-1359322-20170614052501

Also there's this:

"“And I’m their master as well as being a Devil from the House of Gremory, Rias Gremory. My house holds the rank of a Duke. Let’s get along from now on, Ise.”"

- Volume 1

This means the rank may apply to the entire house not just one person.