A family tree on the internet lists this person as adopted.
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Mary apparently died between 1910-1920, because in 1920 Warren is married to Mary's sister, Ella.
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Nebraska obituary index
Kincaid Mary J. Sheldon 1867 1911 Jefferson 01/05/1912
[S2] 1900 Census.
Name: Mary Kincaid
Home in 1900: South Omaha Ward 1, Douglas, Nebraska
Age: 32
Birth Date: Dec 1867
Birthplace: Missouri
Race: White
Gender: Female
Relationship to Head of House: Wife
Father's Birthplace: Ohio
Mother's Birthplace: Ohio
Mother: number of living children: 2
Mother: How many children: 2
Spouse's name: Warren
Marriage Year: 1889
Marital Status: Married
Years Married: 11
Residence : South Omaha City, Douglas, Nebraska
[S54] Cemetery Records Online, Wyuka Cemetery, Lincoln, Nebraska (Reliability: 3).
Kincaid, Ella Sheldon Sec-26 Lot-5999 Sp-5
Kincaid, Mary J. Sec-26 Lot-5999 Sp-9
Kincaid, Warren E. Sec-26 Lot-5999 Sp-8
[S2] 1900 Census.
Name: Warren Kincaid
Home in 1900: South Omaha Ward 1, Douglas, Nebraska
Age: 38
Birth Date: Jun 1861
Birthplace: Iowa
Race: White
Gender: Male
Relationship to Head of House: Head
Father's Birthplace: Iowa
Mother's Birthplace: Iowa
Spouse's name: Mary
Marriage Year: 1889
Marital Status: Married
Years Married: 11
Residence : South Omaha City, Douglas, Nebraska