It is a truth universally acknowledged that if Peter Cushing is in a Hammer film, he must be in want of vampires. But I admit I was thrown off by the initial presentation, in which Viennese twins are hanging out in what I first mistook for the colonial countryside, what with all the British accents and Puritans burning witches.
But eventually it is made clear that there’s a devil worshipping count who lives in a castle and is protected(?) by the Emperor(??), and Peter Cushing is approximately as much of a bad guy as the count is[1], and really the only thing left is to determine why it’s called Twins of Evil when a) one of the twins is pure as the driven snow and the other one is basically just bored with exile to the sticks, and b) it takes almost 90% of the movie for the count’s seductive new protégé to unleash her twins of evil, if you know what I mean and I think you do.
[1] In retrospect this must have been practice for the Grand Moff Tarkin role