Twelve-year-old Chris Wallace and her sister Jackie, seven, whose parents are in the throes of a divorce, dread spending the summer in the Vermont woods (while their mother takes a nursing course) with a grandmother--their father's mother--whom they don't know. For that unusual omission, as for every other circumstance or action here, there's a psychological explanation; but on its own somewhat complex, case-history terms, the book is not unlikable. Chris and Jackie don't know Grandma Wallace because their father ""couldn't stand"" his (recently deceased) brain-damaged sister; as a small Child, he had been sent away to boarding school, while she had been kept at home--only because, he believed, his parents loved her more. And besides, the book will demonstrate, he's a charmer who thinks first and last of himself--while Chris' ""nagging,"" ""boring"" mother is the responsible, ""grownup"" one of the two. The road to this recognition is paved with Grandma's blunt utterances and kindly improvisations (as when she rigs a tent in the attic bedroom to hide the scary bare rafters); and it's capped by the truly dreadful day when Daddy turns up for a much-awaited visit with a strange woman and her three children in tow--children to whom he's more attentive than to ""Daddy's girl"" Chris. The verities are true enough, but the whole affair is as musclebound as the moral lessons of yore.