The book I’m trying to find is from the 70s. The protagonist is a teenage girl visiting her elderly Aunt Rachel for the summer, I think. There’s a handsome man who’s a caretaker, I think, named Ewain? Ewan? In any case, it turns out that he’s hundreds of years old, and he’s really Merlin, and the aunt has some sort of charm that keeps him under her power. He has to do her bidding, and that keeps her young. I think they kidnap a young girl every 20 years or so and all of Aunt Rachel’s physical problems and aging gets transferred to the girl. And the protagonist is next in line. But I think Merlin saves her. Jeez, I read this about 35 years ago, over and over again! I’d love to find out what book it was!