I remember that it was a scifi/fantasy book about a girl whose grandmother or aunt or something buys a large, ornate mirror, old and in need of being resilvered with the phases of the moon carved all around it. The girl is checking out the mirror one night and pushes in one of the carvings and suddenly in the mirror she sees another girl, who pulls her through the mirror (I know, never been done before). The main character finds herself in the creepy house of the creepy mirror girl and her creepy family, which I believe existed somewhere in the past; I remember at one point the main character was scrubbing the floor with a lye-based soap that burned her hands. She spends her few free moments trying to figure out how to work the mirror and get back home, although the creepy girl doesn't want her to go away. The last thing I remember is that just as the main character figures out how to get home through the mirror, she finds out that the creepy family has sold it and it's been taken away.
Does this seem even vaguely familiar to anyone? I'd love to read it again.