User blog comment:B214/DxD Zero: Sirzechs story/@comment-2603:3020:B0F:6700:75F0:1375:A662:BF4F-20180906144955/@comment-31137559-20180908081123

For your last point, it was never said that Falbium had any siblings. Yes we know he's from the House of Glasya-Labolas and that Zephrydor and the former heir of Glasya-Labolas who was killed by the Old Maou Faction are related to Falbium due to being born into the same family but the relation between them and Falbium was never revealed. Like the relationship between Ajuka and Diodora was never revealed.