There seems to be confusion in records with Mildred's last name being either Horlow or Horton. The records on the Minnesota Historical Society state that Ione's mother's maiden name was Horlow, whilst her brother Ray has a mother by the maiden name of Horton. However, I'm fairly certain that Mildred's last name was Horton, as through a census search I've found a Louisa Horton and Charles Horton who match Damarna's ancestry search. In 1905, they had a two year old daughter named Ethel May Horton (Mildred's sister), so Mildred would have been born 2-3 years after. Also tracked down a record for Ethel, with her birth listed as 18th February 1903 in Olmstead County. Still can't find any birth record for Mildred unfortunately or grave or obituary.
MNHS State Census Index
http://people.mnhs.org/finder/bci/1903-18178