Lillian's marriage record gave her mother's name as Christiana:
"Pennsylvania Marriages, 1709-1940," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:V267-BRC : 11 February 2018), Edgar Esher Huff and Lilian Margaret Mccormick, 19 Nov 1902; citing Dauphin, Pennsylvania; FHL microfilm 21,172.
Looking for Christiana, I found a couple of intriguing possibilities.
1900 Census, living as mother-in-law in the home of Adelaide Bach:
"United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M3S7-J19 : accessed 17 April 2019), Chris. M. Mccormick in household of T* Bach, Precinct 3 Harrisburg city Ward 6, Dauphin, Pennsylvania, United States; citing enumeration district (ED) 59, sheet 13A, family 291, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,241,402.
Born about 1836 in Pennsylvania, both parents born in Germany. Also in the home was a Jno L McCormick, brother-in-law, born 1872, occupation Insurance.
In 1880, John and Adelaide lived with their widowed mother Christ, born in Germany in 1834:
"United States Census, 1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MWFS-Z8K : 15 July 2017), Christ Mccormick, 1880; citing enumeration district ED 93, sheet 327D, NARA microfilm publication T9 (Washington D.C.: National Archives and Records Administration, n.d), roll 1123; FHL microfilm 1,255,123.
Unfortunately, no Lillian or Margaret in the home. The 1900 Census indicated Mrs McCormick had six children, and all six are listed in the 1880 Census. It all fits, except for the missing Lillian.