362L: Sister Turns Out to be Another Personality

I’m looking for a YA book from the late 80’s or early 90’s. It’s about a group of school students that go to an old house. The main character is a girl that has a sister. It’s revealed later that the sister is actually another personality. I think there was a picture of a girl looking in the mirror on the cover. I think some bad things happen to the other students, but I don’t remember specifics.