Joe Knox was enumerated in the Phillips household in the 1900 census. There he is called nephew, and is listed right after Mary A Knox, likely his sister.
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.
"Texas, World War I Records, 1917-1920," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:QV18-XJVK : 9 March 2021), Joe Knox, 04 Aug 1918; citing Military Service, Galveston, Galveston, Texas, United States, Texas Military Forces Museum, Austin.
"United States, Veterans Administration Master Index, 1917-1940," database, FamilySearch (https://www.familysearch.org/ark:/61903/1:1:QPTJ-N4KD : 3 September 2021), Joe Knox, 27 Feb 1919; citing Military Service, NARA microfilm publication 76193916 (St. Louis: National Archives and Records Administration, 1985), various roll numbers.