She was still living in 1954, and probably unmarried, being mentioned as Mary Cook, surviving sibling, in her sister Adelaide Cook Daly's obituary.
Sources
↑ "United States Census, 1910," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MKLG-MFW : accessed 14 April 2021), Mary D Cook in household of Chas C Cook, Precinct 8, Washington, District of Columbia, United States; citing enumeration district (ED) ED 158, sheet 8A, family , NARA microfilm publication T624 (Washington D.C.: National Archives and Records Administration, 1982), roll 153; FHL microfilm 1,374,166.
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DNA Connections
It may be possible to confirm family relationships with Mary by comparing test results with other carriers of her ancestors' mitochondrial DNA.
However, there are no known mtDNA test-takers in her direct maternal line.
It is likely that these autosomal DNA test-takers will share some percentage of DNA with Mary: