User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-30924344-20170601105623/@comment-31137559-20170603053246

If there was another sibling, that would make them a rare case among devils. Lord Phenex has four children which is super rare and is a case unheard of among devils. The most devils seem to have is 2 children even if someone like Lord Bael had two wives. Now there might be other not from the main branch(for example cousins). For example Riser is of a side branch of the House of Phenex since he isn't the oldest child and heir. And when he has children they'll be of the side branch as well. Unless something happens to the main branch which is through Ruval.

Leviathan would be referring to Serafall in this case. Since Sona was the former Student Council President and her sister was Leviathan. And someone doesn't need to be of the same family to be a Satan. Why does just a Sitri have to be Leviathan? Satan is eligible for any devil. You do know that Serafall is still considered Leviathan until a new one gets picked? The name has been associated with her so long. You guys are just overthinking things.