Well, I must stand corrected. Your information is exactly right.
I spoke again to my mother and tried to "push" her a bit about some of these details. At that point she remembered she had an old shoe box of some things. In the box was what appeared to be an "autograph book" that had belonged to Kathryn. Among the entries was one from a sister named Lizzie Sullivan and another from a sister named May Y Sullivan (Mary??). Another entry was signed "Your friend and schoolmate, Ursuline College, Feb 21, 1889." Another was signed "your friend Lizzy M Barlow" and another "your friend Anna Barlow, May 3, 1888". Many of these entries were addressed to Katie Sullivan. My mother had been unaware of these other sisters. The Barlow friends certainly locate Kathryn in Grand Forks in the late 1880's.
Also in the box was a newspaper clipping describing the marriage of "Miss Kittie Sullivan to Oscar Mercer, now of Great Falls, Montana". The clipping is a fragment without a date and does not appear to be the same one you discovered. But clearly you were correct about this.
So, the John Sullivan age 53 living in Grand Forks, Dakota Territory, in the 1880 Census must be the father. The brothers aren't listed I suspect because they were older and probably living under a different roof. The John Sullivan in 1870 Census seems less certain.
However, I was able to find reference to an Ursuline school that opened in Grand Forks in Sept 1883, later becoming known as St. Bernard's Academy.
What do you suggest from this point? Obtaining marriage record for Kathryn and Oscar; Kathryn's death certificate?
My mother remembers seeing at some point a Civil War discharge paper for John Sullivan but can't find it now. I know if I had his unit/regiment I could research the regimental history for mention of him. I appreciate any suggestions.
And I can't thank you enough for what you've already found.
p.s. I will visit my mother next month (opposite coast, I'm afraid) and plan to view the contents of this box personally. But other than a lot of photos, she didn't see any other printed material.