The first thing is common knowledge, Vampires can turn people with a single blood sucking session. It takes a certain amount of time to fully convert someone, Or if you want to torture them you dont fully convert them.
I'd have to say its because she has a 10 year old body as well. I could make a whole rambling about how its really just a russian conspiracy out to get me, but that wouldn't really fit well.
Vampires have families, As say Vampire A was turned by Vampire B, And Vampire A turns Vampire C into a vampire, They would all be related as technically they all stem from the same vampire, And any vampire who came from Vampire B would be consindered 'distant family'. There's also the theory that vampires can breed like humans, And its backed up in many places. Choose which theory you will.
Vampires are undead, This is a fact. Zombies and Vampires are classified in the same group, which has a long name but it is generally the undead. For one, Vampires dont survive on blood, They survive on life essence. What happens when all your life essence get's taken away? You get turned into a vampire, And become an animated corpse, aka undead. The title Scarlet Devil is just that; a title. Devil is also usually a word used in old myths as saying 'this creature is not holy and should be killed asap', Which Werewolves zombies vampires and a whole assortment of other things can be classified as devils.
I'm confused here, You think that they may be undead here, But what about last paragraph?
Remi is an immature vampire. And its hinted at that she's weak of mind. She locked up her own sister in fear of what would most likely not happen, She hates being 'bullied' (aka chastised), And she puts up a tough front with her only reasoning for it is to protect what little pride she actually has, and seemingly inflate it tenfold. Remi is, again, weak-minded or just not the sharpest tool in the shed, And there's many reasons how her saying "Coffins are for the dead" could be wrong. Vampires also dont have to sleep in coffins, They just need to sleep away from any sort of light. Technically she's right, But from what you got from saying 'coffins are for the dead' can just lead to more theories by yours truly, or rambling.
Usually devils are strong to fire in RPGs while undead are not, along with many other things.