Mary Knox is enumerated in the Phillips household in the 1900 census. There she is called niece and is listed just before Joe Knox, likely her brother.
Sources
"United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M3GJ-ST5 : accessed 28 October 2021), Dan Phillips, Justice Precinct 3, Guadalupe, Texas, United States; citing enumeration district (ED) 67, sheet 27A, family 451, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,241,641.
For further research
The timing, the location, and the surnames make this record interesting:
"Texas, County Marriage Index, 1837-1977," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FXSH-BH7 : 10 March 2021), J.A. Phillips and Mary A. Knox, 24 Sep 1908; citing Guadalupe, Texas, United States, county courthouses, Texas; FHL microfilm 1,035,127.
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