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.