Brides of Dracula is indeed awesome and a welcome return for Peter Cushing, it was apparently based more off of Carmilla than Dracula.
Not quite. THE VAMPIRE LOVERS starring Cushing is the one based on "Carmilla."
It's part of a separate cycle of "Karnstein" vampire movies that includes:
The Vampire Lovers
Lust for a Vampire
Twins of Evil
was based on "Carmilla," there would be a lot more lesbianism in it!