The book I’m trying to remember is a YA novel about a girl in middle or high school. Her parents were recently divorced, and her father remarried not long after. I think his new wife was a teacher, or maybe a substitute, at the girl’s school. The girl has a new babysitter or nanny or au pair, possibly an older high school or college student. I think they might have held a seance or used a Ouija board together, but I’m not positive. There was a scene with the girl and her mother driving in the rain, and the girl was thinking about her father’s second marriage, with the words “Divorced in (a month, maybe December), married in (another month, possibly May)” running through her head like a chant in time with the windshield wipers. I think the girl’s last name is Beckwith, or something similar, but that’s the only name I remember. I think it was probably published between 1985 and 1991, or sometime close to that.