Headscratchers / Twins of Evil

• Count Karnstein is aware that his family has a dark and enigmatic history that has hints of occult jiggerypokery and even Satanism wafting from it, but he seems remarkably ignorant about the details. He doesn't know that vampirism is part of his family story, doesn't know that Mircalla Karnstein is entombed in his sitting room, he doesn't seem to know anything about vampirism at all, which is pretty odd for a Karnstein. If any of the other films in the Karnstein trilogy are to be taken as canon (dubious, yes) then the grounds of the castle should be literally crawling with the Count's undead ancestors, but apparently that's not the case. You'd have to conclude that this Count Karnstein represents some cadet branch of the family that only inherited the title after the main Karnstein line (based at the Castle Karnstein we see in The Vampire Lovers and Lust for a Vampire) (un)died off.
• The Mircalla Karnstein featured in this film is unlikely to be the same Mircalla Karnstein from The Vampire Lovers for a number or reasons. This Mircalla died in 1547, was blonde and buried in a sarcophagus in the middle of the castle's living quarters, while the other Mircalla died in 1546, was brunette and was buried in the crypt of another Castle Karnstein. Unless, of course, they are the same Mircalla and her presence in this film's alternative Castle Karnstein hints at some unrevealed 16th century backstory where she died in 1546, was reborn as a vampire, and was then buried again at this castle in some sort of occult necromantic ritual and returned to her own Castle Karnstein (from The Vampire Lovers) after being resurrected and turning this Count Karnstein. That would explain the different burial dates and the different locations, and as to the shifting hair colour, well in 'Dracula' Lucy's hair changes colour after she becomes a vampire, perhaps it's an Undead trait.

• At the start of the film The Brotherhood are in a high state of Godly panic because there is already a vampire on the loose targeting local men. This is - before - Count Karnstein’s carelessness with peasant blood awakens Mircalla Karnstein. So who is the other vampire and what are they up to?

• Who killed Anton’s sister? He deliberately sent her to visit their elderly aunt as a ruse to get her away from Gustav’s threats, but somewhere on the way she was killed by a vampire. But which one? Freida seems to find the news amusing, but she typically targets men. The Count? Possible, he does have to feed on someone, but he’s also the type to boast about it to Anton and he never does. The original mystery vampire? Again, possible, but they also seem to exclusively target men. That leaves Mircalla. Has she been hanging around in the area keeping an eye on her protege? It’s a puzzle.