User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-31137559-20170418121057

Just because Rias was marrying Riser doesn't mean that she would leave her household. She is the heiress of Gremory and Riser is the third son of the Phenex household, not the heir. Eneely Vassago is the heiress of the house of Vassago and Iolova Amon is her fiance and he's the second son of his house. And Sona and Seekvaira are the heirs of their house. It's more like their husbands are being married out of their house since females can be the heads of their household. Rias and Riser were engaged because they got ahead of themselves and Zeoticus was concerned about continuing the Gremory line. And he didn't want to wait for Rias to find the one she loved first. He prioritized her being the heir rather than her actually being a person. There can be more than one heir. Rias and Millicas are the only Gremory eligible to be the head of the household and Rias is first because she was born first. If something ever happened to Rias or she steps down, she could just hand over the title of head of house to Millicas.