User blog comment:B214/High School DxD EX/@comment-201.239.133.207-20170104020556/@comment-1359322-20170105012521

Also no Rias' children can't inherit the Head of Gremory position. Rias is just a fill-in for Millicas.

Lord Gremory > Sirzechs > Millicas

That's the original inheritance line. The only reason why Rias was chosen despite not being in the line is because Sirzechs and Grayfia didn't have a child even after several hundred years. So Rias is chosen to fill in that spot till an in-line heir is born.