This is the Texas message board, so it might be helpful for you to post to the Oklahoma board in Hughes Co. Have you ever sent for Katie,s death cert. or tried to get an obit on her? Did you find her on the 1910 census? I found them on the 1920 and 1930 census, after her death, but not before. Do you have either of the marriage records for Charles? The more you can post will help others to help you.
Katie is buried in the Glory Cemetery in Hughes County, Ok. and there are other Roberts and Robertsons buried there, but I could not make a connection to Katie for them. Not sure why she is buried away from her Garrett family.
Do you have the death cert or obits for any of the children, or Charles Garrett, as it might give some clues about their mother?
I am not related, just trying to help.