whose bleeding hand is she licking? my first thought was Suika but Suika wears those chain things in her wrists so...

EDIT: after checking the artist's other works I confirmed that's Sakuya's left wrist.

Are you sure it's not Flandre's hand ? Seems like her wrist cuff...
And, well, she is a vampire, so maybe she's trying to turn Alice into one ?
... If that's how it works in ZUN world. He never said how the Scarlet sisters turned into vampires, did he ?