THE "LINE" in David H. Elias`s Crossing the Line (Orca, 160 pages, $16.95 paper) is the US-Canada border. In the title story, two brothers, Steven and Bill, float down the creek "on the hastily built raft, pretending to be Tom Sawyer and Huck Finn." The line is indicated by a steel marker: "There were some numbers on it, and the words `Canada` on one side and `United States of America` on the other." The stories deal with a family living in a southern Manitoba farming community. Bill is leaving home but Steve has to stay behind with his sister, who is crippled by polio. Trying to teach her how to ride a bike, he thinks that things "would be different if Bill were here ... Bill could do things like that." There are other lines that Steven must cross. In "The Last Visitor," formally the most experimental story, he is incarcerated in a mental institution. His madness seems too contrived, but, that aside, this is a fine chronicle of family relationships done in a spare, understated style. Crossing the Line has an apposite line from Slaughterhouse Five as its epigraph: "Take it moment by moment, and you will find that we are all, as I`ve said before, bugs in amber."