I don't think this is Charlotte, sister to Wilbur. There is no evidence that their mother, Emma/Anna was a Singley. Their grandmother was Christina Singley. This was evidenced by 5 of the children noted from census for Christina listed on the Minnesota Death Index with a mother's maiden name of Singley.
The middle initial for Charlotte as seen in the 1920 census does not resemble an "M" in the slightest. My best guess is "E" or maybe "S"
Maybe we should take this conversation off the list?