User blog comment:B214/High School DxD EX/@comment-201.239.133.207-20170104020556/@comment-10330822-20170105085256

Part of me feels like there is a pretty low birth rate among purebloods, or devils in general. Hence why Lord Gremory and Venelana were married for centuries and only had two children. Likewise, Sirzechs and Grayfia only have one son.

I believe it kind of speaks to why Rias and Millicas are so close in age, and Sona too. Lord Gremory and Venelana "tried" to have another children to replace Sirzechs after he became Lucifer. Likewise, Sirzechs and Grayfia were trying to have their own child after the war ended.

In other words, it took countless years for them to actually have children because the birthrate is so bad. That would kind of explain why Ex is younger than his siblings even though Rias is Issei's first wife, its because Rias was harder to get pregnant because she was a pureblood.

In regards to tradition, it makes sense, the last names are kind of odd.

The entire reason Venelana wasn't allowed to succeed as the Head of Bael was because she was the daughter of a mistress, even though she was the oldest and had the Power of Destruction.

In other words, even if Kurenai was much more powerful than Ex, even though he is also older, it doesn't matter, because Akeno is a mistress.

Of course, as I was saying before, this may all be irrelevant. Chances are the girls are Ultimate-Class with their own territories to rule, so the fact that they aren't apart of Gremory is irrelevant. The children will succeed their mothers and probably be well-established in their own right.