User blog comment:B214/High School DxD EX/@comment-201.239.133.207-20170104020556/@comment-1359322-20170105042203

@Bennia, it's not a matter of gender. It's a matter of inheritance line.

The correct inheritance line is

Lord Gremory > Sirzechs > Millicas

If Millicas was born earlier then Rias wouldn't even be considered for that position. Sona is the same. Her line of inheritance in second to her niece/nephew if she has one. Because the line goes

Lord Sitri > Serafall > Serafall's child.

It's not


 * Lord Gremory > Sirzechs > Rias > Rias' children.
 * Lord Sitri > Serafall > Sona > Sona's chidren.

So Rias and Sona's children aren't even considered for the clan head post from the start since they were just temporary replacement for their elder siblings.