Hi, Just a thought - Have you searched Family Search Org online
https://familysearch.org/Do you know her Place of birth in the USA?- there may be some information re Mildred Rhea McKinney - b. c 1900 in USA -
Census will show names of parents & include any siblings still living at home with parents.
Do you know When /Where the Marriage to Arthur CLARK occurred.
Registration of Marriage - Long Form Marriage Certificate will show names of both sets of parents including maiden names of the mothers etc.
Perhaps the family?
From Alberta GenWeb Local History book site - online
CLARK, Mildred; Cameo Continues; Sturgeon
Have you searched Cemetery where Mildred Rhea McKinney is buried - perhaps Arthur CLARK is also buried there - Cemetery office can give you dates etc
OBITUARY for Mildred and Arthur - as well those for the children - may show names and locations of other family members - who may be able to assist with your quest.