Request #64

I probably read this book in the early 80s and I think it was new. A teen girl living near Gardiner, MT is the main character. Her family has horses, maybe a ranch. Her 16th birthday is coming up, and her older sister Althea (I think) disappeared when she was 16. The main character’s father is suddenly encouraging her relationship with a boy. Her mother is kind of upset about that, because she knows why he’s encouraging it.Turns out that the women in the family are given a choice at 16: to go on with a normal life or join some kind of magical society living deep in the forest. I’ve googled everything I can think of and can’t find this book! Suggestion: Becoming Rosemary by Frances Wood