Found: YA: Group of children travel into a mirror

A group of kids, 4 I believe find a tall mirror in an attic and are able to pass through it, becoming invisible and exploring while "in the mirror". They begin to realize that the longer they stay in the mirror world the harder it is to get back. Eventually the reader is unsure if the children return, or if their doubles from the mirror dimension have taken their place in the "real" world. Thanks so much for your help, want to read this one with my daughter 20 years later!!

I added one to the list after searching worldcat.org.
Stine, Let's get invisible!
https://www.worldcat.org/title/lets-get-invisible/oclc/53358774&referer=brie...

Wonderful! It is Stine: Let's Get Invisible. I searched Google several times amd never got close, thank you so much!

I searched worldcat.org for invisible mirror fantasy, and there it was. You can restrict by juvenile fiction book, too. It's great when they are so easy to find when the clues are available to search! Enjoy.