Does anyone know who her parents might be?

In all census, she states her father born Indiana, her mother in Illinois.

1900, Nancy 1st marry's Ira Allen, South Dakota
son, Ira Levi Allen born S.D.
2nd, by 1910- William Whitehead, South Dakota (soldier)
sons Hutchinson Whitehead, Stansbury Whitehead, daughter Olive Whitehead (all children born S.D.)
1920 Montana census (widow) 3rd, James W. Plumlee
1930 Montana census. James W. jr. Plumlee born Idaho.