Here is a 1915 census record from South Dakota for Ole and Mari Finnestad's great granddaughter, Maine Olson, living in Pleasant township in Lincoln County.
I could not find any other family members that were part of this particular census, but everything matches. Maine was born January 19, 1899 so she would have been 16 in 1915. Maine was also born in Illinois and her parents, Mary (Knutson) and Stephen Olson were both born in Norway.
I'll have to do some additional research to find who Maine was living with during this census which may provide some clues as to why she was living in South Dakota.
Maine's grandparents were Elizabeth (Finnestad) and Ole Andreas Knutson.