Have checked cemeteries in Canadian County and adjacent counties and do not find Mary Devine. She may be unmarked if buried here. I checked the City Directories and found this
1903 - Mary E. Devine received her mail from Geary, OK which is west of El Reno and this matches your record of her living in Valley Twp.
1912 Mary E. Devine is shown living at 815 Bickford in El Reno
1918 Mary E. Devine is listed as a widow of Laughlin Devine still living at 815 Bickford
Checked the 1920 Canadian County Census and she is not listed. Maybe she moved to be with a child somewhere else or moved back to Ohio.