User blog comment:B214/Season 3 confirmed!!!!!!/@comment-24.104.16.162-20150329163637/@comment-1359322-20150401004330

@anon, it's not only Rias, but it applies to Sona as well. The child of the older sibling would be the one to take over the clan. Why do you think Millicas is named the next clan heir after Rias, even before Rias became clan head, because he's rightfully the one in line after Sirzechs. Be noted even though they are Aunt and Nephew, Rias and Millicas are about 13-14 apart in age. Rias is only 18 while Millicas is about 4-5 years old.

Rias was only nominated because like you said, Millicas wasn't born back then when they nominated Rias, and due to the low birth rate of the Devils, they probably thought that the next heir won't be born so soon, so they had nominate Rias. But now that Millicas is born, Rias only needs to serve as the Head of Gremory till Millicas reach adulthood and can take over as Clan Head.