User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-29963364-20170418021819

If Rias was the "ture heir/heiress" then the Gremory Clan wouldn't have been so willing to give Rias away to the third son of the Phenex Clan, Riser.

It's likely that they made this engagement after Millicas was born. They agreed because even if Rias left the clan to marry Riser, they would have Millicas (the first-born child of the Original Heir, Sirzechs).

But in the future, Rias does become the Head of the Clan, so she's not a simple "place-holder." She is just "option A."