She is listed in the 1870-1880 censuses, but by 1900 her mother is listed in the census as having given birth to four children, only one of which was still alive. That living child was Mary's older brother Charles Benjamin Dunnam, who is single and living with his parents in that census.
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&db=lrtate&id=I4827