The teenage girl protagonist doesn't like her stepfather, but has to live in his house. She finds out through visions caused by portraits and/or mirrors that he's a murderer. Eventually a person (the murdered person?) swaps bodies with her to take revenge, temporarily putting her consciousness in the portrait. I remember a single line from the novel, "A mirror that has witnessed a death can be a fearsome thing," but don't recall what character said it to her.