I can't believe I hadn't read this!!! I am a big Mrs. Piggle -Wiggle fan, but I hadn't discovered Nancy and Plum until now.
The two loving, smart, imaginative sisters remind me of Meg and Jo March-- Nancy is 10 and a very capable seamstress and cook; Plum (Pamela) is 8 and a tomboy who dreams of becoming a cowgirl. They are orphans living with the dreadful Mrs. Monday and her horridly spoiled niece Marybelle at a boarding house for unwanted children. (Their rich bachelor Uncle John is quite oblivious.)
The book takes place over the course of a year, stretching from a Christmas Eve alone in the cheerless boarding house (and then locked out-- so the girls hunker down in the barn), through a year of little triumphs and disappointments, to a very happy, better-than-they-ever-imagined Christmas Eve in a loving home with dear Mr. and Mrs. Campbell.