The Magic Half by Annie Barrows

The Magic Half is a great book. Eleven year-old Miri is the middle child between two sets of twins. When she is sent to her room for smacking her brother with a shovel, she notices an eyeglass lens taped to the wall of her room. After she looks through the lens, she finds herself transported back in time to 1935. Miri then realizes that she is standing in front of a girl whose name is, as she later finds out, is Molly. Molly is also eleven and convinced that Miri is a fairy and has come to save her from her vicious cousin and her cruel aunt.