User blog comment:B214/The one-shot answer ranting/@comment-120.20.47.163-20170422142026/@comment-30924344-20170422144108

Yes Rias is the heiress, but due to Rias marrying Issei and having a child with him, her inheritance line isn't passed on because half-bloods can't become the Head of a clan, so when Millicas comes of age and is ready to take over, Rias will step down for keeping purity in her clan, while gaining a much higher position due to Issei. Millicas himself will be expected to marry a pure-blood devil and have a child with said woman, but he can find romance in a reincarnated-devil or "commoner devil", as he is allowed to have a harem and it will be fine as long as the legal wife is pure-blooded.