SOLVED. 90s YA book about twin girls and a mirror [s]

[Lizzy]

YA book about identical twin girls involving a mirror. One of them is sucked into the mirror and into the past, while the other one has to pretend to be both sisters. They live at some kind of boarding school or dormitory. The twin at the dormitory place has to figure out how to get her twin out of the mirror and it's tied to the moon somehow.

I read it in the 90s. It could possibly be part of a series, but I can't recall. The twins were definitely identical. I think the cover had a blonde girl on the cover and a very ornate mirror.

[bookel]

http://www.librarything.com/list/554/...
Twins fantasy

http://www.librarything.com/list/551/...#
Twins science fiction

Stranger in the mirror (Phantom Valley #4) by Lynn Beach
http://www.librarything.com/work/1043...
That it? Could be fantasy I think? I really ought to read my copy...! It has come up multiple times on other forums.

[Lizzy]

Yes, Stranger in a mirror is it! Yes...thank you!