User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-23853100-20170419124658

@Bennia, do you even know what you're talking about? If Millicas was born before Rias, Rias wouldn't have been chosen.

The reason why the Gremory needed an heir is because Sirzechs and Grayfia didn't give birth to one yet. That's why Zeoticus and Venelana had to try to give birth to another heir themselves, which they did and that's Rias but that rule applies only to Rias. Remember Sirzechs' son is still a Gremory, thus he is entitled to the seat being the son of the Eldest Son. B214 even quoted you didn't he?

"Heh, so he’s next in line after Buchou. Certainly, he’s the son of the family’s eldest son. Sirzechs-sama may have left the family, but his son is an heir of the important Gremory family."

Did you even read this quote properly or did you just skip it because you don't want to accept the truth?

Sirzechs is the eldest Son and the one entitled to the clan, however, he had to surrender it due to his job. But while  he (Sirzechs) may have left the clan, but his son (Millicas) didn't, naturally Sirzechs' right is pass down to Millicas .

Rias' right was handed to her in the absence of her brother due to a need for a new clan head. It's to help fill a void. 200+ years without a child, the Gremory doesn't know when Sirzechs will actually give birth to an heir. They can't just wait for another 100-200 years to see if Sirzechs and Grayfia will have a son or daughter or not. They require one promptly, and that just happens to be Rias.