On my new favorite website findagrave I may have found the cemetery where Cordelia, Edaward and Opal are buried.
This is all guessing but a Cordelia J. Pope born 1867 died 1951 is buried in Riverview Heights Cemetery Kennewick WA
In the same cemetery are Edward J. Engstrom 19 Dec 1910 to 23 May 1997 (and his wife) and Opal L. Hansen b. 9 Nov 1895 (we know Opal L. Pope was born in Nov 1895 from 1900 census)
When I looked up the SS Death Index records for Edward and Opal they were born issued in Wyoming. Opal lived in Puyallup WA when she died.
I can't tell if these graves are together in the cemetery but the stones are of a similar style. Cordelia's is labelled Grandmother, Opal's says Mother.