I read a paperback book from my junior high school library in 1989. It’s about a girl and her father meets a pretty woman who has a son or daughter too (can’t remember who moved into the area or how they meet). The youngsters become friends. The father falls in love with the woman, but the girl gets suspicious of her – she notices that the woman is a bit strange. She discovers photos of her looking exactly the same in past decades. She finds out the woman is a witch and does not age, the witch’s son or daughter doesn’t either.  The witch had been dating, marrying, and killing the husbands after a few years because they see that the woman and kid stay young. The girl tries to prove to her father that he is in danger and tries to stop him from marrying her. I think the book is set in either the south or in New England, I remember fog and time playing a role in the story.