Have you considered that the listing in the 1880 census for Albert Sisk (Line 26) might be the enumerator's take on "A. L. Sisk? He might have been called "Al."
"United States Census, 1880," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:MXKX-DZV : 19 February 2021), Albert Sisk in household of A J Sisk, Cottage Grove, Saline, Illinois, United States; citing enumeration district ED 112, sheet 165A, NARA microfilm publication T9 (Washington, D.C.: National Archives and Records Administration, n.d.), FHL microfilm 1,254,248.
That would make him the Ab [illegible] Sisk (Line 41 - b. 1874) of the 1900 census:
"United States Census, 1900," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:MSHX-QPW : accessed 27 March 2021), Ab Sio Sisk in household of A J Sisk, Cottage Township, Saline, Illinois, United States; citing enumeration district (ED) 79, sheet 5A, family 81, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,240,342.
Albert A. Sisk, as an individual in the Tree cited on Abraham Lincoln Sisk's WikiTree profile, has no sources other than these two censuses. Perhaps he never existed apart from being a census-created alter ego for Abraham Lincoln Sisk.