Ennis's getting the postcard and putting up the postcard above the shirts on the wall of his trailer and his oath of "Jack, I swear--" was the beginning of Ennis working through his own handling of grief and bereavement due to the loss of his apparently first best friend who was also loved Ennis with no changes in whom he was.
In the short story as Diana Ossana first read it and had Larry McMurtry read in the New Yorker Magazine, there was nothing about a married daughter at all. But, I don't have a problem with Alma Jr. going to her father to tell him that she was going to get married and Ennis asking, "Does Kurt love you?" In the movie, she did not have to drive so many miles to the rural trailer park somewhere near Riverton; but, in the book, Ennis's trailer was not even in the same county and it was on the Stoutamire ranch. I actually had a little tear when Ennis asked that question in the movie.
We are sorta OT here; but, I have known many guys who were born to be great fathers, yet they failed miserably as husbands or should not have been married in the first place. My late partner/husband, Ed, had been married 13 years to a woman whom he loved as a friend and she already had a son and a daughter. His step-daughter, Linda, was 17 when she herself got married just before Ed divorced Jessie. Linda told me that Ed was the only father she ever knew and he could not have been a better father.