User blog comment:B214/Talk About What You Want! 2/@comment-74.77.52.225-20140801025045/@comment-1359322-20140815171627

One thing, just because Issei marries Rias doesn't mean he's the Head of the Gremory, the Head of the Gremory will be Rias not Issei. Of course the servants will have to give the proper respect to Issei, but Issei won't have any authority over the Gremory, only Rias has that authority. Issei can help in decision making and such but in the end it all resides on Rias not Issei.