It’s a mystery/thriller, with some paranormal overtones. I was born in ’66 and I believe I’d read it by the time I entered high school in ’80, so it must have been written sometime before then. Possibly way before then, as I recall the book was a beat-up paperback when I picked it up.
There’s a love interest for the cousin – I want to say he’s a lawyer? And that he also had some kind of relationship with the dead cousin? Maybe?
I think the dead cousin’s name is Cathy or Kathy, or maybe it’s the surviving cousin’s name? I don’t remember how the book develops the ghost angle – whether the ghost is real or just the cousin’s imagination – but I’m pretty sure it turns out she was murdered.
There’s a supporting character who’s a friend of the living cousin. I remember she shows up at one point and is described as an earth mother type, I think, and says something about the coffee she’s offered – like “heavens, yes, keep it coming” or maybe “I like my coffee like I like my men” but I could be wrong on that.