Among Baum's siblings, four survived into adulthood to be his contemporaries.
- Harriet Alvena Baum (1846–1923)
- Mary Louise Baum (1848–1933)
- Benjamin William Baum (1850–88)
- Lyman Frank Baum (1856–1919)
- Henry Clay Baum (1859–1916).
Harriet married William Henry Harrison Neal, who later became a successful tax assessor in New York. Baum dedicated his fourth Oz book, Dorothy and the Wizard in Oz, to Harriet.
Mary Louise was Baum's favorite among his siblings; he used her name more than once in his works. (See Mary Louise; Annabel; Louise Merrick.) Baum dedicated Sky Island to her. She was an aspiring poet, but never achieved much success. She married Henry D. Brewster, a businessman who was, at various times in his life, New York State Assessor and head of the state's volunteer fire service.
Benjamin became a chemist and was active in the Baum family lubricant business in the early 1880s. Unfortunately he died young.
Frank was closest to his younger brother "Harry" when they were young; they worked a printing press together to publish a family newspaper. Baum dedicated The Magical Monarch of Mo to him. As an adult Henry Clay became a physician, one of several doctors in the Baum family over multiple generations. He cared for his mother Cynthia during her final years, and married only after her death. Baum dedicated The Emerald City of Oz to Harry's daughter Cynthia, later Cynthia Luise Dattan Baum Tassini.
The Baum family encapsulates the revolution in infant mortality that occurred at the end of the nineteenth century. The survival rate among Baum's siblings was barely 50% — but among his children it was 100%.
The four of Baum's siblings who died young were:
- Cynthia Jane (1843–48)
- Oliver Stanton (1844–48)
- Edwin Clay (1853–56)
- George McCellean (1861–63).
- Sue Ferrara. "Dr. Baum: The Royal Historian's Brother." The Baum Bugle, Vol. 44 No. 2 (Autumn 2000).
- Katharine M. Rogers. L. Frank Baum, Creator of Oz: A Biography. New York, St. Martin's Press, 2002.