User blog comment:B214/DxD Zero: Sirzechs story/@comment-2003:45:5D00:9D00:D59D:7785:ABEF:4D2C-20181006195431/@comment-31137559-20181009211237

You're assuming that there are any branch families(not a part of the main inheritance line) of the House of Gremory. It's not out of the question for Rias to have some distant cousins.

Well devils have low birthrates if you haven't realized.