With the first name you provided, I have located Josephine and Martin's graves. It's at Lakewood Cemetary in Minneapolis.(www.lakewoodcemetery.com)Kate is there too.Here are the listings off the website.
JOSEPHINE O. KOON Age 70 Death Date 06/18/1922 Place of Death Unavailable Disposition Date 06/21/1922 Site Id 159-16-25
MARTIN B. KOON Age 71 Death Date 08/20/1912 Place of Death Unavailable Disposition Date 08/22/1912 Site Id 159-16-24
KATE KOON BOVEY Age 89 Death Date 07/17/1964 Place of Death Unavailable Disposition Date 07/20/1964 Site Id 159-16-16