User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-1359322-20170418031140

"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."

Rias is made heiress out of necessity. @Pawn, Rias is 18 while Millicas is around 7-10. Also, the main reason why Rias is chosen as head is because:

"“Even if you say that, Issei-kun seems to have surpassed my power already, so isn’t it enough already? I think it will be fine even if I settle down soon.”

“Please wait until Rias has graduated from high school at least before talking about retirement.”"

Zeoticus wants to retire, and doesn't want to wait for 2 more decades. >_>