''Everything is relative except relatives, and they are absolute.'' This was a frequent lament of the photographer Alfred Stieglitz, who took some of this century's most memorable pictures, published one of America's most visionary art magazines, and ran several of New York's finest galleries. It is fittingly quoted by his grand-niece, Sue Davidson Lowe, in this biographical study of her renowned ''Uncle Al.''
Although Stieglitz liked to portray himself as ''a loner . . . working in isolation and basically against the mainstream,'' Lowe points out that he chose to spend much of his life in the bosom of a large, overbearing extended family. ''He seems to have needed them,'' she writes, ''if only to rekindle his rebellion, to fuel his purpose.''
Lowe traces Stieglitz's career from his student days in Berlin, where he first falls in love with the camera, through his unhappy first marriage and his later intense relationship with painter Georgia O'Keeffe to his crotchety but productive latter years. The book is sprinkled with vivid reminiscences, many dating back to Lowe's childhood at the family summer home in Lake George, N.Y., where Stieglitz spent part of almost every year and where his ashes were buried.
''It is my hope,'' writes Lowe, ''that I have succeeded in humanizing Uncle Al, whom I loved but did not idolize.'' In this, she succeeds admirably; she is less successful in conveying what impelled Stieglitz as an artist.