User blog comment:B214/Season 3 confirmed!!!!!!/@comment-24.104.16.162-20150329163637/@comment-73.178.133.185-20150401003227

I think there has been some misunderstandings here...you too B217. IRL royalty and nobility, the inheritence of a certain position goes through a "line of succession", generally amongst males first. For the head of the House of Gremory, the line of succession was the eldest son/child Sirzechs first and the only othe child of the current Lord Gremory, Rias, second. Most likely Millicas was not even born yet when Sirzechs gave up his rights of succession in order to become Satan, otherwise he might have supplanted Rias in the order. Once Rias takes up the headship of her family officially,  her children would normally take precedence over Millicas even though Sirzechs was originally meant to be head. The biggest issue is that Rias and Issei's children won't be pureblood -- but recall, Issei is a "special" existence and his kids might be given an exception. Besides, the current trend of the Devils is moving away from such traditions, hence why Sairaorg has been seeking out the 'exiled' 72 pillars who'd committed that "taboo".

On a side note...someone stated somewhere  that the children of reincarnated devils would be reincarnated devils, but that's wrong. They'd simply be non-pureblood devils.