Library book, remembered as Orbit series, but it could have been Dangerous Visions or New Dimensions or even Year's Best SF. This was mid-60's, 1965-69, and the book was fairly new, possibly paperback. The story was about vampires, whom I believe were brother and sister, living in a Victorian mansion in San Francisco, where I moved shortly thereafter. (I don't believe there were any straight horror anthologies then, thus its inclusion in a sci-fi anthology; maybe they were alien vampires.)
No recollection as to the author, although I had read a lot of sci-fi and horror by then. Wish I could remember more details. I just remember its being a really good, atmospheric story that would probably make more sense to me now after living in SF for so many years. I would love to read it again. Any help appreciated.