User blog comment:B214/DxD Zero: Sirzechs story/@comment-2603:3020:B0F:6700:75F0:1375:A662:BF4F-20180906144955/@comment-2601:8C:601:5692:BCB0:2900:8048:4D81-20180912132518

No, while it's not explicitly stated that the heir that was killed by Diodora was Falbium's sibling, it is heavily implied that this is the case considering Falbium's previous position and what happened in the others' cases. Of course, there's always the chance that a different cousin was that heir, however, it seems likely that such a detail would have been mentioned, at least in passing, in connection to the current circumstances with Zephyrdol. That such interesting information wasn't including lends to the greater likelihood that it was a sibling and that Ishibumi simply forgot to point it out since he/she was already dead.