I've gone back on Ancestry and put the question marks in. I think I was working on that tree late one night and went to bed and didn't get back to it. I had put a comment on the tree warning that all the information did not seem to match up, but the comments don't show up very readily on Ancestry.
I don't know anything about Susan. My guess is that she may have come to the U.S. alone as a young woman, since I haven't yet found any Peterson or Petersen relatives living near the McCarthy family. All the information I have is from online research.
My boyfriend's parents are dead. He is descended through the oldest McCarthy daughter, Nellie, who married Christopher McDonell in Stevens Co., WA. Their son, Monte's grandfather, Patrick McDonell, moved to Butte County, CA, by 1930. Monte was close to this set of grandparents, but doesn't know much about anyone further back. (His genealogy is fun to work on, though. Mine just goes: farmer, farmer, farmer, etc. back to before the Revolution!)
Patrick's grandmother, Susan McCarthy, and Aunt Gertrude McCarthy Cogdill were living in Fresno County until the late 1920s, when they moved to San Francisco, where Susan died. I don't know whether the extended families were still in touch then, though.
I've e-mailed the N. Dakota university library that has the database online to see whether they will look up the two persons listed with the family in 1885 who were not named McCarthy or Peterson. They may have been staff of the hotel, but maybe they were family members.