It is stated in various areas that Robert never married. I have not found where he married.
The 1900 Census shows the family in Texas , Robert's father is functioning in some way as Law Enforcement , 2 prisoners are shown in the 1900 Census with them and the area is listed as Justice One.
He passed away in 1929.
Sources
Name Robert Thomas
Event Type Census
Event Year 1900
Event Place Justice Precinct 1-2, 5 (part), Knox, Texas, United States
Gender Male
Age,20
Marital Status Single
Race White
Race (Original) W
Relationship to Head of Household Son
Relationship to Head of Household (Original) Son
Birth Date Jun 1880
Birthplace Texas
Father's Birthplace Alabama
Mother's Birthplace Alabama
Household Role Sex Age Birthplace
John P Thomas Head M 54 Alabama
Alpha A Thomas Wife F 54 Alabama
Marlin L Thomas Son M 31 Alabama
George O Thomas Son M 23 Alabama
Robert Thomas Son M 20 Texas
Ada E Thomas Daughter F 17 Texas
Dora L Thomas
Daughter F 14 Texas
Ice P Roberts Prisoner M 30 Alabama
William E Dudley Prisoner M 31Texas
"United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M3PW-4V6 : accessed 12 June 2018), Robert Thomas in household of John P Thomas, Justice Precinct 1-2, 5 (part), Knox, Texas, United States; citing enumeration district (ED) 43, sheet 2B, family 42, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,241,651.
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DNA Connections
It may be possible to confirm family relationships with Robert by comparing test results with other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
However, there are no known yDNA or mtDNA test-takers in his direct paternal or maternal line.
It is likely that these autosomal DNA test-takers will share some percentage of DNA with Robert: