Hi Harold, This is the only Mildred Renish that I found searching Ancestry and Family Search.
Frank Renish, head household, b. NE, 44, machinist railroad. Mabel, wife, b. IL, 34. Frances, b. MN, 13. Olive, b. MN, 11. Mildred, b. SD, 8. Lois, b. MN, 5. Adelbert, son, b. MN, 1 2/12ths. Leroy, b MN, 3/12ths.
I haven't done any research in South Dakota but most states now have Digital Archives. I'd use Google and enter South Dakota Digital Archives.
Do you have a copy of George's obituary? There might be something in it that provides more clues as to where Mildred was born.
If you don't have it, you can find a volunteer at Random Acts Genealogical Kindness. Go to RAOGK, read guidelines, choose Multnomah Co. and one volunteer.
A Mildred Townsend b. 1 March 1911, Last Residence=Portland, Multnomah Co. died Dec. 1983. Her soc. sec. # was assigned in Oregon before 1951. Source: Social Security Death Index.
To ensure this is your Mildred, request a copy of her obituary from RAOGK. Successful Searching!