User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170420000500/@comment-29963364-20170421144307

Rias can inherit the clan because, even if she marries a Reincarnated Devil, she is still a Pure-Blood Devil. But since her and Issei's children won't be Pure-Blood Devils, they can't inherit the clan. Rias would have to pass the title down to the only remaining Pure-Blood Devil in the Gremory Clan, Millicas.