SOLVED. Children's fantasy - Girl finds her missing twin. [s]

[Audrey]

I read this when I was elementary school, so ~10 years ago. It's definitely a children's book.

The main character is a little girl with four siblings: two sets of twins. She feels like she should have had a twin, but she doesn't have one. One day, while exploring their old house, she stumbles across an eyeglass lens taped to the wall (or something like this). When she looks through it, she ends up in a kind of alternate version of her house, where she finds another version of her family and her missing twin.

Plot happens, and I believe she and her twin live happily ever after.

Any help is greatly appreciated! Thank you!

[Ayshe]

The Magic Half probably?

[Audrey]

That's the one! Thank you!