I remember it was some sisters, don't know how many. They all had cats. I remember the main woman sacrificed her cat or her sisters cat to Hecate. And there was a man, possibly brothers, that were like opposing forces? And I believe the main woman was in love with the main man.

I think there was a scene where they were in a body of water for some reason.it was a dark series, I read it about 15 years ago in high school. I don't think it should have been in a high school library, it was brutal.

That's all really vague. But the most important part is that she definitely sacrificed a cat to Hecate. No amount of googling seems to help me though.