When sis has to be mom too; The Night Swimmers, by Betsy Byars. Illustrated by Troy Howell. New York: Delacorte. $7.95.

Retta Anderson had cared for her two brothers, Roy and Johnny, even before their mother's passing. Now that they're on their own, Retta has full-time charge of the boys. Their father, Shorty Anderson, is a country- and-Western singer intent on fame and fortune. He is not a mean father, or even an uncaring one, but he is exceedingly negligent, leaving his children home alone at night while he works.

Retta is determined that they won't "grow up ignorant" and is pleased with her ability to find interesting things to do for free, like being secret "night swimmers" at a neighbor's pool. But suddenly Retta's carefully controlled world begins to crumble. Johnny finds a new friend and acts distant and rebellious. roy, torn between his brother, whom he idolizes, and his sister, whom he loves, finds refuge in food and fantasies. Retta feels unappreciated and frightened by her strong feelings of resentment. The tensions among the siblings culminate in a near-tragedy. No one is hurt, though, and the event helps remind Shorty of his responsibilities. And Retta, with the helpful understanding of Shorty's girlfriend, Brendelle, is able to sort out some of her confusion and accept the fact that independence does not always mean rejection.

The author, who has written many popular books for young people (including "The Summer of the Swans," which won the 1971 Newbery Award), is especially adept at portraying families and children in crisis. The Anderson family is no exception, and the strength of the book lies in its skillful characterization of Retta, Johnny, and Roy -- each thoroughly real and recognizable.