Margaret Durbin Harper Sibley was born in 1851, the only child of Fletcher Harper and Margaret Durbin, and the granddaughter of Fletcher Harper, founder of Harper Brothers Publishing Company. On February 6, 1873, she married Hiram Watson Sibley (1845-1932) whose father founded the Western Union Telegraph Company. They continued to live in New York City for many years. Their first child, Margaret Harper Sibley, died there in 1874.
Mrs. Sibley was active in charities both in Rochester and in New York City. She was a charter member of the YWCA board in New York and its first secretary. For more than 45 years she was a member of the board of supervisors of The Genesee Hospital and for 25 years was board president. When she resigned in February 1929, she was made honorary president for life. She was the first president of the Nurses Training School board and continued in that office until elected president of the board in 1904. Along with her husband, she provided the funds to erect a building known as the "Brothers Cottage" for treatment of contagious diseases. This building was dedicated to the memory of her two children, John Durbin Sibley and Hiram Sibley, Jr. who died of diphtheria. In 1916 they remodeled the building to be used as a laboratory of pathology. Mrs. Sibley built the first children's ward and furnished it and also the first nurses' home. In addition to this, the greatest expenditure was for the Sibley Pavilion, that housed many of the hospital's private patients.
She took an interest in the training school and its pupils that continued long after she resigned as president of the board. In the early days, when the number of nurses was small, she often made a practice of becoming personally acquainted with each young woman entering. She provided the Margaret Harper visiting nurse from 1891 to 1915 in memory of her mother.
Mrs. Sibley died on May 28, 1939, at her home in New York City. She also maintained a home at 384 East Avenue in Rochester. At the time of her death, at age 87, she was survived by her three children, Ruth Sibley Gade, Margaret Urling Sibley Iselin and Fletcher Harper Sibley, former president of the United States Chamber of Commerce. The week of her funeral saw the lilacs in bloom, the Memorial Day parade and the graduation of the class of 1939 from The Genesee Hospital School of Nursing.