User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-10330822-20170417222442

If I recall correctly, Rias is the proper heir, and unless she gives her title up, Millicas won't receive it under any condition.

Sirzechs left the family to become Lucifer, it was part of the requirement to become Satan. As a result, Zeoticus and Venelana had Rias, and she became heir. Only after Rias was born, was Millicas born.

As a result, because Sirzechs was passed over, Rias is the heir, and Millicas after her. Sirzechs lost his right to inherit, and thus, so did Millicas, hence why it goes to Rias. When a Prince leaves his kingdom, his future sons don't inherit in his place, because titles are passed from father to son.

In short, has Sirzechs had Millicas BEFORE becoming Lucifer, Millicas would be the heir, but Sirzechs isn't technically Gremory anymore, so the title is going to Rias. Now, because of the bloodline, Millicas is next after Rias.

What EX said was simply that Rias gave the title Millicas, it never said it had anything to do with Millicas being the proper heir, Rias just did it. Most likely, like I said above, because Issei has an even higher standing than Gremory does in the future. He is an ambassador, but he may have other titles as well. For all we know, he could have been made a "Great King" like Bael.