Is baby sister necessary?; Fanny's Sister, by Penelope Lively. Illustrated by Anita Lobel. New York: Dutton. $7.95.

Nine-year-old Fanny's world is not likely to be that of most of her readers: Not only does she live in Victorian England, but she is matter-of-factly surrounded by Nurse, Cook, Nellie the kitchen helper, and Hobbs the groom. Even her problem, on the surface, seems wondrous: Her new sister is more than a new sibling; she is the eighth child in a family that grows "as regularly, it seemed to Fanny, as Christmas and Easter and birthdays."

The focus is not really the new baby sister, but Fanny herself, a child who is learning to cope with the changes every new year brings. The recognizable "mew" that tells Fanny one morning that another baby has arrived opens the floodgates of relative reality: Another year gone, she is even more excluded from the world of the Young Children (who are cuddled on laps and go for donkey-cart outings) and is progressing through the world of the Old Children (who struggle with lessons and must go to church).

Fanny's crossness is a temporary defense, as is an earnest prayer in church for a cherry tart with clotted cream, followed by a rebellious wish that God take back the new bay. When at dinner the truly unexpected cherry tart materializes, Fanny panics at the thought God is answering all her prayers. Too embarrassed to tell stern Papa or even preoccupied Nurse of her dilemma, she decides to run away. She ends up at the vicarage, where she tries clumsily to convice the wise vicar that she wants work as a kitchen maid. The vicar is a magnificently realized character: His patience in letting Fanny work her own way through her dilemma until the proper moment for him to intervene is the best message of the book.

Fanny grows more and more endearing and her world more and more captivating as the story unfolds. The language is absolutely delightful, with natural but rich imagery and the perfectly chosen expression. Anita Lobel's illustrations are particularly nice in that they help the reader visualize an unfamiliar world and unfamiliar words (a goffering iron and a coal scuttle, for instance). "Fanny's Sister" bears further witness to its already-distinguished author and illustrator.