For the life of me I cannot remember the title of the book or the author. It was from the late 80’s or early 90s and was a mystery with teenaged characters. There was a girl sent to spend the summer with her aunt and a selfish cousin. I think a few years pass after the cousin’s death and there is a reunion and all of these odd things start happening. It was on a lake and had a French name like Fond du Lac or Chance du Lac. It might have been on the Wisconsin/Canada border. There was a romance element to it and the heroine was meant to feel like she could trust no one.
Stump the Bookseller
New format, same book stumper solving.
I believe whatever happened to the cousin involved a jetski.
I am pretty sure it was a Scholastic book fair book.
This sounds like a Lois Duncan novel, perhaps “Summer of Fear”? In that one, the heroine is the one whose cousin comes to stay, though, after her aunt and uncle are killed in a car accident. Because her parents are dead, the heroine, Rae, has to be nice to the cousin, Julia, who initially seems very shy, but later turns out to be moody and very selfish and manipulative. She alienates Rae’s parents and flirts with her brother. (I had a feeling she maybe turned Rae’s boyfriend against her as well, but I don’t see a mention of a boyfriend in the Wikipedia summary.)
It’s set in Albuquerque, but I do seem to remember that there’s a lake and Julia appears in a bikini (and her appearance gives a hint that she’s not who she says she is, because dressed like that, she seems older than fourteen or fifteen.)
I had forgotten this, but it turns out that there are supernatural elements like the ones you mention and eventually it’s found that “Julia” has been performing witchcraft against the family.
https://en.wikipedia.org/wiki/Summer_of_Fear
Sounds close but I am pretty sure the heroine went to stay with the aunt and cousin up north in Wisconsin area.
I was able to find it and apparently I was totally wrong on the cover. The book is Girl in the Shadows by Miriam Lynch.