According to the 1880 census, Elmer was Stillman & Rachel Jane [Rose] Niles' son.

1880 Census, Fountain, Juneau Co., WI, E.D. #185, p. 369A-369B, Dwelling #108, Family #121, enumerated 18 Jun 1880:

Stillman Niles, 40, Farmer, b. Wis, f/b NY, m/b NY
--- Jane, 36, Wife, Keeping House, b. NY, f/b NY, m/b NY
--- Carrie?, 10, Daughter, At School, b. Wis, f/b Wis, m/b NY
--- Elmer, 5, Son, At School, b. Wis, f/b Wis, m/b NY
--- Jesse, 3, Son [sic], b. Wis, f/b Wis, m/b NY
--- Prentice, 1, Son, b. Wis, f/b Wis, m/b NY

On his WWI Draft Registration Card, Elmer's full name was given as Elmer Jason Niles and his birth date as 21 Dec 1874. Mabel Niles of Mauston, Juneau Co., WI, was listed as his nearest relative,
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On the 1900 Census [New Lisbon City, Juneau Co., WI, E.D. #84, Sheet 3A], Jane reported she was the mother of 12 children with seven still living. In 1910 [Lisbon Town, Juneau Co., WI, E.D. #68, Sheet 8B], Rachel J. repeated the same count.