User blog comment:B214/Season 3 confirmed!!!!!!/@comment-24.104.16.162-20150329163637/@comment-73.178.133.185-20150401213400

What the above two said. Rias became the heir because Millicas was not born yet, otherwise line of succession would have had him first. Now that Rias is heir, unless she gives up her rights, loses it for some reason or dies before succeeding her father, Millicas won't become head. Once Rias becomes the new head, the line of succession would change to prioritize her children unless there are reasons to deny their position. That is how 'line of succession' works in real life (and unless Ishibumi states otherwise, it is the only basis we have). The issue of Issei's children being anle to succeed their mother is as I previously stated.