User blog comment:B214/New anime 2017 discussion blog/@comment-97.86.117.132-20161230234256/@comment-1359322-20170101015541

Regarding the head of the Gremory topic, the actual line for that is:

Zeoticus > Sirzechs > Millicas

But
 * 1) Sirzechs had to forfeit because he became Lucifer
 * 2) Millicas wasn't born yet.

Because of these two reasoning Rias was chosen as replacement till Sirzechs has a son or daughter. Which ironically happened few years later, so Rias' duty got shortened.