The Magic Half (Miri and Molly) Unabridged edition

Miri is the non-twin child in a family with two sets of them - older brothers and younger sisters. The family has just moved to an old farmhouse in a new town, where the only good thing seems to be Miri's ten-sided attic bedroom. But when Miri gets sent to her room after accidentally bashing her big brother on the head with a shovel, she finds herself in the same room...only not quite. Without meaning to, she has found a way to travel back in time to 1935 where she discovers Molly, a girl her own age very much in need of a loving family.