^Nez - Are Vampires considered undead in Japanese mythos ? They count as a type of youkai in touhou right? That sounds more like they are a race that are alive rather than undead- and at least some are born vampires rather than having been bitten - though I dunno if Remi & Flan were once humans in cannon but I'd assumed not.
The Scarlet sisters are western style monsters, so Japanese mythos really aren't relevant. I don't think their creation was ever addressed in canon in any case, just that they're originally not from Japan and the fact that they're vampires.